Answer
This question was answered on November 11, 2004.
Much of the information contained herein may have changed since posting.
First, Check your scanner manufacturers support web site for new XP drivers.
In the general case, Windows 2000 drivers probably will work fine in XP (no guarantees about specific cases, but the two systems are pretty much the same, and it works for me and others).
Windows 9x drivers won't work in XP. If you installed XP over Win9x, then you must uninstall the 9x scanner driver and install the XP driver, they are different.
SCSI scanners in XP require that you add ASPI yourself, since XP and Win2000 do not install ASPI like Win9x did. See Microsoft Knowledge Base article Q300674. For Adaptec cards, search Adaptec for the ASPICHK.EXE test program to tell you what version you have now, if any. There is a new version of 4.70 ASPI for XP on www.adaptec.com site, (and this package includes ASPICHK.EXE). A few scanner installations will install ASPI, but many do not. ASPI is the interface between the scanner driver and the SCSI driver (much like TWAIN is the interface on the other side, between the scanner driver and the application program). The symptom of no ASPI for SCSI in WinXP or Win2000 is that the scanner is NOT detected in any way by the scanner driver. If it sort of tries to work, then the scanner drivers is seeing the scanner, meaning that you must have some version of ASPI already. Run ASPICHK.EXE to make sure what you have.
Some older scanners are supported natively by XP (and WinME). The WIA Twain drivers (Windows Imaging Acquisition) are installed by XP, and will appear in the File - Acquire menu of your photo programs. See the list of XP WIA supported models (also at Microsoft Knowledge Base Q293356). If your old scanner is supported, WIA will provide a minimal Preview and Scan capability (Scan mode and resolution can be specified). BTW, WIA does not need ASPI to access the SCSI driver, it uses other methods already in XP.
Hope this helps. Good Luck!