Odd X31 symptoms

[hoodedmonkey3]

hi everyone. hopefully, someone can help me out with this odd problem i'm having. it's a tad long though so no worries if you don't wanna read it.

anyway, it's either a system board issue or a virus scan issue or both... or something else.

The problem:
computer is ultra slow when I boot up with the power plugged in. Computer runs fine when I don't plug in the power and start, then after the computer loads, I plug in the power cable.

When computer is started with power adapter plugged in:
mcafee scans the hell out of everything and consistently hogs 99% of system resources when i do anything. right click, open an app, file, etc. even when nothing is running. when i disable OnAccess scan on mcafee it's ok. so it does seem like a mcafee issue. but why does this happen only when AC was plugged during startup. But its a bit weirder.

A few weeks ago:
my fan died. When i replaced it, the computer wouldn't start when the AC adapter was plugged in. After unplugging the cord and starting it on battery, it would start. This continued for several days. Now, it boots either way. So perhaps there is a system board issue as well?

some other info:
Case I: (AC plugged in during boot)
Idle CPU temp: low-mid 50s. (mobile meter)
Idle CPU speed: periodically goes to 1.2ghz then back to 600mhz.
During Active usage:
temps almost reach 70
Fan turns on and off all the time
Active CPU speed: mostly in 1.2ghz

Case II: (AC not plugged in during boot)
Idle CPU temp: low 40s.
Idle CPU speed: stays at 600mhz.
during Active usage:
temps stay in low or mid 50s.
Fan rarely turns on
CPU speed: mostly stays at 600mhz but occasionally 800, 1ghz or 1.2ghz depending on what i'm doing.

system specs:
x31
1.2ghz
768MB
what else...
XP pro
Mcafee VirusScan Enterprise 8

So... i'm confused. any suggestions? or should i just replace the board. i do have another year on my warranty...

thanks for taking the time to read this. any replies would be greatly appreciated. have a nice day everyone.

- mas