If you full-screen the VM window, in the title bar along the top there's a "USB" menu. Assuming the scanner is attached to the host, it will appear in the menu as an attachable device, and of course you have to attach it in the VM. The VM session will detect the new device, and from there you install drivers and software as usual.
I wasn't able to get a target program configured in XP's Control Panel ~ Scanners and Cameras interface, but if you use the ScanGear Toolbox utility that goes with the scanner model then it's moot. Editing within XP Mode is silly - it makes far more sense to map a drive letter from your XP mode session to your host filesystem, save scans to the mapped drive, and rename and retouch from the host OS.
The first thing I scanned (at 150dpi color) was my tag from the 2011-09-25 Baltimore Great Prostate Cancer Challenge 5K. The second (at 300dpi) was a keepsake photo of Pam and Mike's wedding party. Viewing that full-size in GIMP I saw there was a significant amount of dust on the platen or the photo itself, so I wiped both down and rescanned and Facebooked it and now it has 15 "likes" and counting.
In short order I digitally immortalized my copies of Dan Vena's "Retina Soybean" prints and a page of notes exploring geometry of hexagons. Then I bogged down in categorizing what was left in that box.
Something I didn't consider is just how many "scraps" there are to process. Biz cards, postcards, letters, greeting cards, movie and concert ticket stubs, memo pad take-aways. Just sorting through them is mentally tiring. Another issue is how to preserve both sides of postcards and such. I guess I'll have to work out a process of stitching together scans of each side.
After a mental break, I ended up just arranging all the tix stubs on the platen and scanning in one pass. Then I started scanning old photos. I quickly gave up on separating a platen's worth into individual files; that's a project for another day, if ever, because I intend to have ALL my photo prints scanned and backed up before bed.
There are a lot of pretty crappy shots, as a couple rolls of film jammed in my camera, and I pushed the limits of the film and camera in low light elsewhere - but there are a few stand-outs and of course the memories make it all worthwhile.