I'm running Win XP Professional. SP2 and security updates all up to date as far as I know. I have a 250 GB HD in 3 partitions. ~100, ~100, ~50. OS on the first one.
Anyway, after using the PC in the morning with no problems (crashes, freezes, whatever), I turned it on in the afternoon and it would not boot into windows. It went to the screen that offers Safe Mode (x3), Boot from last config..., and Boot normally. No matter which one of those I choose, it restarts. If I choose Safe Mode you see the files listed on the screen and then it reboots.
I have my old HD still in PC with XP Pro also installed on it. When I open "My Computer" in there, I see the 3 partitions. The 2nd and 3rd partitions are viewable, with labels and total/free space listed. The 1st partition, which is oddly labeled the I: drive (instead of D: or E

shows nothing in the margin for total and free space. When I click on it, it says its not formatted and asks if I would like to do so now.
The last thing I did that morning, if it helps, was watch some videos on Google Video, which I do often.
If I had to settle for just being able to get some files off of it and then reinstalling windows, that would be ok. I'll check back in the afternoon and again tomorrow morning, any help would be greatly appreciated.