After toying around with my HP Pavilion s7520n (Slimline) PC, I ended
up messing up the MBR to the point where the only thing that would fix
it was a Windows reinstall. After backing things up, I grab a copy of
Vista Home Premium (the OS that came preinstalled on the machine was
XP Media Center '05 w/ Service Pack 2 + Rollup) and install it
(because at the time, I couldn't find the OEM disc). After installing,
everything appears to be fine (minus the expected driver issues
because of Vista), but not long after, the system begins to have
issues-it's slow, some applications take a while to load, and startup
takes a while. Mildly irritated with these problems, I find my XP
Media Center disc and reinstall it. But upon reinstall, things are
worse. For a few minutes after booting, the system runs quite
normally. But not long after, it gets quite unresponsive, if I open
Mozilla Firefox, it will continually freeze and unfreeze, I can't open
Task Manager (and after a while, any programs) through any means
necessary because it gives me an error (the application failed to
load), if I log out, it gives me a different logout screen than
before, and soon, the only way to get things running back to normal
(just for a few minutes) is to just give the computer a hard reboot.
Yet, I once toyed with Linux, and I had no problems whatsoever-it's
only for Windows.

So what can I do?

More specs:
AMD Athlon 64 @ 2.0 GhZ
1GB of RAM
250GB hard drive connected to mobo via SATA
DVD-R/RW drive connected via PATA/IDE