This could be a million and one issues. Here's what it's likely to be:
1) A virus or spyware running in the background. Update Windows, and run whatever antivirus and antispyware they have installed after updating those too. AVG and Adaware are good free antivirus and antispyware programs, respectively.
2) They have a load of programs set to start when the computer starts. Remove them from start menu->Programs->Startup folder, so that you just start Windows, and not a ton of other things.
3) General Windows slowdown. Happens to all Windows installs after a while. Solution: backup your stuff and reinstall Windows.
4) Less likely, but run defrag on the drive.