This is one of those questions that are difficult to answer. I believe that the WinXP help and support files say you can (Win2K here at work, so can't look) BUT they have made a mistake with it, as you can't do it the way they say.
You don’t say what type of network you have but as DieSse and Marvin have pointed out the easiest way is to save the scanned image to a shared drive.
I have set up a scanner to only work on one machine but the scanned image automatically saves to a shared drive "learnersshared$ on 'svr1' (L:)". This allows everyone who is logged onto that account to have full access to the image.
This question was asked some time ago in here and the answer was that "yes you can with third party software". Have tried to find the thread but have had no luck. I think it is something to do with SCSI, but it's that long ago I can't remember what it was. Sorry.
I have two PCs that sit in the same room networked. They share a printer and each others hard drives quite happily. But it was a little annoying having to move the 6 feet to use the other PC to scan a document. ;)
Guess I'll just have to consider it good exercise. :)