I suspect that these problems haven't just happened, but it has been getting worse over a long period of time? It probably started off with little things that you just overlooked. It's always best to jump on - even - the smallest problem when you first notice it. Because you can now see how it get's out of hand if left!
I think it's got to that stage that the only sensible thing to do would be to reformat the HDD and do a clean install of WinXP. I believe that is what the repair shop will suggest.
So it you have anything on your PC, of either a personal or delicate nature? You should remove or delete it, before taking it to the shop. If you have 'Memory Sticks' a 'Slave Drive' or an external 'HDD' use these to save anything you want to keep. Because when they reformat you will lose everything, the PC will come back like a new PC.
As a matter of interest what Spyware, AntiVirus and FireWall are you using. When was the last time you cleaned out the history and defragged?