Well, sometimes i do prefer to know the root of the cause, cos reinstalling can be troublesome especially with 2CDs to mess with (and i do like the sense of adventure of finding out wats wrong) Sometimes it doesn't work no matter how hard i try and here i come for solutions. Now the main thing (I'm asking for help):
Some of these things are easy enough to figure out but if you've tried all the usual options without success it's much quicker to Reinstall. Saves a lot of headscratching too.

I actually has a similar problem as YAK52 but mine is it does not show any errors at all, it just revert back to the desktop. I was still flying it fine yesterday and today it just refuse to run. I have absolutely not touched anything (no new addons or anything). Any idea whats wrong?
Try rebooting your PC first. That can fix many of these problems & it's surprising how many people don't even think of doing it. If not I suspect you changed something even if it has nothing to do with CFS2. If you can think of anything you changed, no matter how unrelated it appears that could be your answer. It could be an automatic Windows update or something like that.
I can't think of anything else offhand but I will do my best to help. I'm somewhat handicapped these days as I have no knowledge of WinXP.