This will check the system files and the drive, though if other software has installed and runs properly, it may be McAfee not getting on with any of the recent Microsoft Updates, or anything which was installed when the problem showed up
Click start then run, type sfc /scannow then press enter, you will need the XP CD
System File Check will display an onscreen blue progress bar, when the bar goes, reboot.
Click start then run, type chkdsk /f /r then press enter, type Y to confirm for next boot, press enter then reboot.
Windows will appear to load normally then either the monitor will show progress or the screen will go blank, do not disturb this. This will take an hour or so before it gets to the desktop.