Mmmmmmmmm. could be many a thing.The only way to resolve this is by process of elimination, but we really need to know more info. What os are you running..Me or 98?Don't sound like xp.
What mobo is it?What graphics card have you got? Give us as much info as you have.
1.Start by ensuring that your ram is ok.Test in another machine. I doubt this is the problem tho but you need to make sure.
2. is your psu at least 300w and of good make.
3.When does it reset?
4.Have you done a clean install?
We're ready to help.