DFU problems seem to fall into 3 categories.
a) DFU drive never appears
b) DFU drive appears but then disappears after a few seconds
c) DFU drive appears but cracks up when a file is uploaded to it.
I use Win7 64 most of the time and haven’t experienced a or b, so those are not intrinsic to Win7. They could be driver problems or in the case of b) there may be some other software which is trying to do something to the drive when it appears (virus scanner, file sync program, media scanner etc). I guess the latter is more likely for people’s Win7/8 installations compared with XP. DFU isn’t really a drive although it is presented that way on Windows and can be a bit flaky if any other than a regular file update is performed and that could cause it to disappear if anything else is trying to access it. If trying different ports and checking USB cable haven’t helped then two extra strategies here would be to try to get the driver to re-install (uninstall in Device manager) if it can be seen. It might also be worth booting in Safe Mode and seeing if it works there as this could help diagnose if other running programs are causing the problem.
For case c), which I have experienced, the issue for me was with just some particular hex files that DFU objected to (e.g. PAWN11). Re-organising the short hex records near the end fixed that for me, but I couldn’t work out why.