|"as i have to much information on it that i didn't get the chance to back up before it started playing up and i can't replace any of it"|
First step, slave the the hard drive to another computer ( you cannot get a HD with XP on, to run as master on a different MB ) & then recover all your important stuff, including emails & address book.
I suspect the power supply. Try another from an old comp. no need to bolt in, just unplug the the suspect one, & plug in the test one. Make sure the test one has the same connector first.