Parallel port scanner

A couple of years ago a friend of mine bought me a Mustek parallel port
scanner.
Using this scanner has locked me into Windows (95/98 & NT).
The other day I tried using VMWare and set up my 1st parallel port as
/dev/parport0 (I do have a /dev/lp1) and installed Win98 and the scanner
software in the virtual Win machine--no problems.
When I scanned a picture, straight lines were pixilated & weird. Overall
quality was poor.
When I scanned the same photo of Tux using native Win98, the picture was
sharp and clear.
Anybody know why running a VMWare Win98 and scanning would produce somewhat
corrupted results rather than running it in native mode?
Could this apply to other things as well, such as data on a hard drive?
Mark