The cpu and mainboard is buried pretty deeply down. I have the service manuals for it, and show that the cpu is upgradeable, however I'm not sure about what kind of chips you can put in. The second memory slot is, as you say under the...
Installing XP Pro isn't too bad. You can get the original drivers here:
http://support.acer-euro.com/drivers/downloads.html
I had some problems figuring out the AMD power now driver, but all you have to do is go into Device...