Here are some of my favorite ones.This one is for the CPU
For users of Windows 2000 or XP you actually have to tell the OS what size L2 cache your processor has otherwise it won't be used properly! Luckily it's fairly easy to fix.
Click on the Start button then go to run. From there type REGEDIT and press the "Ok" button. After it opens the Registry Editor follow this path. hkey_local_machine, System, CurrentControlSet, Control ,Session Manager, Memory Management
Once you're in the Memory Management folder look for the DWORD value
Right click on that and go to modify. Very important you have to change the Base from Hexidecimal to Decimal before modifying the values. After that's done just punch in your L2 cache size so if you're using an AthlonXP since the CPU has 256KB L2 Cache you enter 256. For Duron's enter 64 and for P4 Northwood's enter 512.
After that's done you should notice your programs now have quite a bit more zip and the system is less sluggish because Windows now knows how to use your L2 cache properly.This one is for the memory
While Windows2k and WInXP manage its memory much better then the Win9x based operating systems, you can make both the actual Win2k or XP OS run more efficiently by keeping all of its code in system memory (since it's the most used program) rather than in a HDD swap file.
Go to Start -> Run and type regedit. From there follow this path. HKEY_LOCAL_MACHINE -> SYSTEM -> CurrentControlSet -> Control -> Session Manager -> Memory Management. Right click on the DisablePagingExecutive DWORD value, change its value to 1, save and reboot.
Now Win2k and WinXP will stay in your system memory instead of going into the HDD swap file. Please note, you must have at least 512MB of system memory otherwise you'll notice a slowdown in performance!