I have a PC I built a little over 2 years ago, and it has worked fine until about a few weeks ago. The first oddity was that after I turned it off, it would turn on by itself about 5 minutes later. I didn't think much of this and would just turn the master power switch off in back so it would stay off. This worked fine for a while, but then after I would flip the master power on, it wouldn't always turn on immediately. For a while, the internal speaker would repeatedly beep twice, be silent for about 30 seconds, and beep again. It would do this for about 10 minutes and then stop, and I'd be able to turn it on. After a while it kept getting longer until I could turn it on (up to 1-2 hours). Now whenever I turn on the master power, there is occasionally a long continuous beep, but when it stops I can't turn on my computer. The front panel lights come on and the post LED displays what I interpret as "8." (which I haven't been able to find a meaning to). I'm thinking this is related to my power supply...any ideas?

My specs are:
fatal1ty AN8 SLI mobo
BFG Geforce 7800GT
2 x 1 gig ram (just some generic if memory serves me correctly)
Real Power 550W (can't remember exact wattage)
XP