Replacement is always easier; however, I'm not sure that is completely necessary. I'd start with the hard drive. If you can, connect your hard drive to a friend's computer as a slave drive and then make a disc image of your hard drive. Acronis True Image is the software I use to do this task; there is a trial version, I believe.
After you've made a disc image of your hard drive, format it, and then use Acronis to reimage your hard drive. If that still doesn't help you, then i'd look into possibly replacing the hard drive.
Bottom line is, I feel a little reluctant to suggest replacing the mobo and CPU if you can still get through POST and the OS loads (somewhat). This problem seems more like a hard drive issue than a mobo/cpu issue.