Like other posters, when my iMac was using Snow Leopard, my iMac could see and read folders and files on my Windows XP laptop, and vice versa. However, since installing Lion, my Imac can still see and read folders and files on the laptop, but I cannot do the same in the other direction. However, the laptop still sees the iMac in "workgroup computers" and Mshome but the access stops there. Selecting the iMac from "workgroup computers" brings up a login window but, after entering my iMac user name and password, I get an error message that ends with the really useful information that a Remote Procedure Call error has been caused. After this, the small login window does not appear again. I can still print from the laptop to the printer connected to the iMac.
This may be a long shot and way off beam, but soon after installing Lion, I created a second user account for my wife to use. If I make any changes to Preferences - Sharing in the main account, this seems to cause a problem with fast user switching as, when changing to the second account, the screen freezes in a light grey blank and the only recourse is to restart the computer. Could the addition of a second account on the iMac cause some sort of problem with sharing from a pc?
Whatever the cause of the problem that so many seem to be experiencing, I hope that a permanent fix that non-IT experts can apply can be identified.