Xp Failure

I started a defrag on my Pc last night but to my shock and horror my pc locked up at only 1%done. I restarted and was more shocked to find it did not start. It got to the start in safe mode prompt then locked up again. I therefore told it to boot from disc and it tried but failed as it said I had no operating system installed??? It wanted to do a new install and format my drive first. I switched it off and gave up as I did not have a clue. I formatted the Hd originally in NTFS before I installed windows the last time and my friend thinks this and whatever happened with the defrag could be why windows appears not to be there and the disc appears empty. I did not have any way of getting on line to query what it could be last night so switched it off till tonight and am now using the girlfriends laptop for help. Anyways switched it on tonight and it booted to windows but ran like a dog so I decided to do a repair. It booted into dos (is that what you call it) asking for me to tell it what to do so I typed help and got all of these instruction to type at the C: prompt. Don't know what they mean. So having a brainwave I just booted back to windows and told it to do an upgrade, all went fine it loaded as expected but thenwhen it got to installing windows it froze at only 34% and there it sits at this very moment after an hour. Can anyone help me? have I lost everything? Off for a wee cry just now!