If you've got a windows xp cd lying around, you might want to consider creating a boot disk using
UBCD 4 Win. I always have a copy of this lying around in case of emergencies. You can also use the disk to run a series of tests and fixes, apart from being able to extract all your files.
As far as iTunes goes, like djdannyp said, you can just remove all authorised computers and re-authorize them. I suggest that, one you do have a fully working system, to keep an eye out for a windows version of Requiem. It's a tool that removes the drm layer from iTunes protected music without any quality loss (i.e. no analog conversion, just drm removal). I am completely drm free now, thanks to this tool. It works on videos and music.
If you have any questions about anything I've said, don't hesitate to ask
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