Arkroyal, you've probably already done the deed so this may be too late. But did you consider cleaning out the registry and then a registry compaction and then a defrag.
My machine had slowed to a crawl, I did the above and it made a BIG difference, especially the defrag. (My system disk was not so much fragmented, more like shattered
)
Probabl not as good as a reformat and reinstall, but A LOT less hassle.
AA