Crashing is usually caused by things such as hard drive failure, chip/memory problems, operating system failures, spyware/adware, virus/infection, etc...

If it won't boot up at all, find the original program discs, and run everything offline/safemode, to try and get it back to a good restore point. Restore points are various date/time restoration backups done by the pc.. Im assuming the pc was backed up at different intervals?

If its just periodic crashing... loose connections in the rom chip, memory, ram problems, adware/spyware, overheating issues..etc..