Reinstalling Windows will sometimes fix the problem. (You can do an 'on top' installation, which retains all of the programs and data on the PC. There's no need to format the hard drive first).
If your friend has got the Windows 98 disk (or you can get hold of one), here's what to do:
First you need a boot floppy. Your friend may already have one but, if not, you can make one by going to
Start > Settings > Control Panel > Double-click Add/Remove Programs > Startup Disk
Put the boot floppy in its drive and restart the PC.
Select 'Boot with CD support'
Watch the screen to see which drive letter is assigned to the CD drive. It will almost certainly be E (not D as it probably usually is).
Insert the Windows disk into the drawer.
Type 'E:setup' and hit return. (That, of course, assumes that the letter E is assigned to the CD drive)
Then simply follow the on-screen prompts.
An on-top installation of Windows never does any harm (although it's sometimes necessary to reinstall some types of antivirus programs after doing it) but it often speeds everything up and it can fix problems like the one your friend has been experiencing.
Chris