I can't prove it, but I've heard it said that Win-7 is more efficient than either XP or Vista.
I run Win-7 and XP on the same PC, but from different hard drives and I see no appreciable difference between them.
My XP is 32 bit and my Win-7 Ultimate is 64 bit. Everything runs fast on my PC.
So if you can reinstall Win-7, you should be OK.
Good Luck,