As others have mentioned, dust leading to overheating is a common problem. I should know! If you have a source of compressed/blower air, such as being able to swap hoses around on a vacuum cleaner, stick it one of the openings at the rear of the tower. Sounds drastic but I know of a well-known Perth private school that does this routinely.
Another suggestion is P2P programs. They have a reputation for causing computer crashes.
