The symptoms you describe could be malware related or they could be due to hardware or overheating problems caused by a failed processor fan, bad memory (RAM), failing power supply, underpowered power supply, CPU overheating, motherboard, video card, drivers, dirty hardware, etc. If the computer is overheating, it usually begins to restart on a more regular basis.
Is McAfee able to remove what it has detected? If not, try doing a scan in "SAFE MODE
If that does not help, then download and scan with Dr.Web CureIt
. Follow the instructions here for performing a scan in "safe mode".
Some rootkits can trigger BSODs, shutdowns and error messages so I suggest you also download and scan with AVG Anti-Rootkit
In Windows XP, the default setting is for the computer to reboot automatically
when a fatal error occurs. You should be able to see the error by looking in the Event Log
See "Extract troubleshooting info from Windows XP BSOD error messages
" and "How to Find BSOD Error Messages