Hi Kim:
In case you are still wating for a response, I had the same problem when I converted my desktop to XP a few years ago, but I was able to get it working. The other day, my USB hub fell off the desk and the 4200's USB cable came out. After I plugged it back in, I could not get the scanner to work (got a message that the computer couldn't find it when I tried to lauch the scanning application), even though all diagnostics said it was installed and working properly. Unfortunately, I could not remember how I got it working a few years ago.
After much trial & error over the last day, this procedure seems to have worked (in this exact order):
1) Unplugged the scanner (both USB & power cord)
2) Uninstalled scanner software
3) Installed downloaded new scanner software (that you already have)
4) Shut down computer
5) Reattached power and then USB
6) Restarted computer
7) With scanner on & attached, ran the XP update (sj700en.exe)
8) Restarted computer
Let me know if this works for you. Sounds like you were halfway there. The key may be the XP upgrade (small program) and the order of steps.
Good luck.
wellsie