Well, you have to have license info first and....it's a bit complicated to explain. The system restore may have an option to put it back to "factory settings". This kind of stuff varies by PC a bit (many have a restore partition. You'd also need any upgrade CD's or files or licenses for stuff purchased afterwards).
Basically, it's not something I'm comfortable helping with since I can't see your screen and don't have all the up-to-date stuff like a computer store will.
If it's a software problem, and you talk to someone in person at a computer store and bring it in, they may be able to help you (check their reputation first online).
But, if there's a hardware problem of some kind, you'd still be unhappy after paying them $$$$.
My guess is the hardware is OK and the software is messed up. But there's a lot of upgrades to install and ....well...you'd want a very fast connection to do that too.
Best to either decide to "take it in" or decide to get a new one (donate the old one to charity/family, or sell it, after removing the disk drives...don't let it just collect dust and get old.)