Help me Figure out whats wrong.

Well, during spring break my motherboard died on me. I can't explain it, it just pooped out on me. I replace it with a similar model. I had to reformatt because of new mobo.Then my hdd starts to become flaky on me (after I installed mcaffee, long story, bad results). It makes me reformat, works for a few weeks, then gets to a point where i have to reformat again. Happens another time but this time only lasted a few days. Then I ask a few guys and they say its a dying hdd. So I backup the other partitions and buy a new hdd. Install Xp and what not and all programs. lasted 3 weeks tops. Refused to boot up during finals week so I threw in the towel.

Now I'm off and was headed to buying a new laptop and disgarding the old system. But after discussing with parents they suggest to buy a replacement desktop for a year then hand it down to my bro then i buy a new laptop for college. However, my bro's computer is also becoming flaky, so i'm trying to fix mine.

Any way, the issue that always comes up is this. I am forced to reformat because when I try to get into windows there is a little black underscore that flashes in the left hand side of the screen. and it stops there. I try to chkdsk but it says one or more unfixable errors. I try fixboot but it gives me some other error. Sometimes i can't even type in fixboot or chkdsk. But when I bought the hdd I made sure it was was good because i rant he diagnostic after I isntalled xp on it and it had no flaws. I made no modifications or changes to it at all nor to windows the night before it happened.

Anything else it could be?
(some people point to psu what do you thing? and how do i check?)
Specs:
Proc.: P4 2.8 stock
Mobo: Asus P4P800 E-Deluxe
Ram: Corsair Pc3200 1 gig (2x512)(haven't ran memtest on it but pretty sure its ok)
Video card: Radeon 9800pro 128mb stock
PSU: 500watt not sure of company, havent checked.
Drivers were all updated.

Recap: One mobo dead, 2 hdd appear to be dead; cause Unknown

Thanks for reading upto this point I kno it was long

Answer to karizma's post; sorry I worded it confusingly, what I meant was:

Everytime it would break down was when it would refuse to boot into windows. I wouldn't get the xp screen with the little bar going. It would post and then there would be a black screen with a blinking underscore on the top left. But after the reformat it would work fine for a while and then screw up later.
I'm going to try and start up my computer again to see what exactly happens, its been like 2 weeks.

Edit: I tried to start up system and this is what it said:
Windows could not start because the following file is missing or corrupt:
\WINDOWS\SYSTEM32\CONFIG\SYSTEM

You can attempt to repair this file by starting windows ... using the oriiginal Setup Cd-Rom.
Select 'r' at the first screeen to start repair.

i typed the error in google and it told me to go into the recovery console and type fixmbr. Only problem I cant find my xp disc! the serach continues.

The reason why i believe its a hardware issue is because the installations keep on becoming corrupted. Some people say it might by a psu gicing badd wattage to the hdds, others say it might be a overheating issue that makes the hdd info corrupt. I doubt it overheats because it doesn't freeze or anything.

One more thing to add, the hdd is supposed to be faster but when I installed it it seemed slower.

true and yes it is, i've ran memtest86 for about 5 hrs, 14 passes and 0 errors. Just to elimante memory. Im desperatly trying to find my xp cd, this is the forum i read it at.http://www.short-media.com/forum/showthread.php?t=24057

I went to microsoft's site and they had a whole series of steps that tries to solve this issue. I tried printed them out, searched for the xp cd all day. When i finally found it, i tried to run recovery system, but when i try to run chkdsk, fixboot or fixmbr and it says can't find volume. When I look at partitions it says 15 gigs of freespace where c is supposed to be but its not there anymore. wat's you're take on it?

Edit: the diagnostic I would like to run is on ym psu. I have been told by many that that is the problem. I don't think the harddisk is the proble, cause when I bought it i ran the utilities to make sure it worked.

In terms of overclocking, no, I've never overclocked anything, everything is stock. I'm trying to figure out why it deleted my partition. Like I said I haven't touched it at all for weeks, and I'm 99% sure I didn't delete it myself. Yes, I can reformat but it will only happen again. This is the second hdd I've bought. If its dead i'm going to send it to maxtor for refurbishment or something it was like 125.

Yes, the 15 gigs is where the c drive used to be but i'm trying to figure out what happened to it, everything in bios seems to be fine. I'll run the hdd diagnostic on it to make sure it's alright.

Well. drive failed, with diagnostic code d395646f, i'm contacting maxtor at this moment and rmaing the drive

it will fix the issue but only for a limited time, this is the second hdd that died on me. The first one died in a similar way except it kept clicking and then one day it stopped booting up.

And before I started to install the software on this hdd i ran all the diagnostics on it to make sure it was fine. and they were, it passed every single test. Now it fails on the first one.