What's the OS, the CPU and RAM?
Replies above cover most of the bases, I would think.
Computers that have been in use for a while can end up with a lot of programs starting with Windows. 90% of them don't need to. (53% of all statistics are made up on the spot.)
Another candidate is a software confliction. Does it do this all the time, or just when some programs are open?
What AV, Firewall and antispyware is installed?