Make sure it is not trying to download Windows updates on a slow broadband connection. This can often happen in the background without the user knowing it. If you think it is, then switch the PC on and leave it for say half a day with nobody using it to let all updates download. You may be prompted for a reboot during this time.
Also make sure none of the disks are full up. If Windows has no room on a hard disk to create temporary files this can make Windows grind to a halt.
How old is the PC?
What version of Windows?
Any idea of the spec of the PC (CPU, memory, hard disk size etc)?