Athlon 64 running very slow (WinXP-pro)

Asus K8V-X Socket 754 Via K8T800 ATX
AMD(newcastle) Athlon 64bit 3000+
Sapphire radeon 9550 256DDR
1 x Crucial 512 PC3200 DDR400mhz
Ebuyer 500 watt psu
western digital caviar 80Gb hard drive.
Windows XP professional



Upon initial install this worked perfect. Installed F-Secure
anti-virus,
though not updated as not on internet at this time. Then used it
running
farcry for 3 days burned a few DVDs, stable no problems and very fast.
Took
it round to my friends updated the Anti-Virus then installed service
pack 2.
AARRGGHH! All of a sudden its very slow to respond, to do anything. Did
a
quick reformat re-installed windows, nope still very slow to respond to
any
comand. Click on folder etc and you wait 5 to 10 seconds if not longer
for
it to open.Installing games is slower than an old 333mhz computers.

Can anyone help because I'm well flustered here.

have you tested the hard disk for problems?
is your CPU usage always high in Windows, regardless of what you're doing?

don't u think it might be a memory resindent virus. Just remove all the partions you've created in the machine and recreate them again. Do the installtion again and install sp2 and teh windows spyware software after that you can fix into teh net. BTW I forgot the virus guard is also essential