Sometimes XP Home systems may have trouble sharing to/from an XP Pro system. I don't know what to expect of an XP Home system trying to access Vista (or vice versa) or whether it makes any difference if the Vista version is Home/Premium/Ultimate/Stardestroyer.
So while you might have trouble getting the Vista computer to browse the network to the XP system, we can try a more direct approach: CMD tools. Perhaps your computers see each other from the commandline.
Try using both the computer name and IP address. Since we know the laptop (XP) can see the desktop (Vista) OK, do this on the Vista. You'll need to know the computer name and IP for the XP machine.
Start > Run > cmd (enter), then execute:
C:\> net view \\mylaptop
A successful command will list something like this:
Shared resources at \\mylaptop
MyNetwork
Share name Type Used as Comment
-------------------------------------------------------------------------------
MyShare1 Disk My Photos
MyShare2 Disk l337 w4r3z
MyCDrom Disk CD-ROM
MyPrinter Printer
The command completed successfully.
If that doesn't work, try the IP address (get this from the laptop's wireless connection status dialog):
C:\> net view 192.168.0.2
If one of those works, then you can try to mount the share directly, like so:
C:\> net use Z: \\mylaptop\MyShare1
.. which should mount the laptop's MyShare1 share to your local Z: drive. You can do the same thing from My Computer > Tools > Map Network Drive in a more GUI fashion.
Users and permissions and passwords can be gotchas here. If you can see the computer with net view, but can't mount the drive, it's likely a permissions problem. Things to check are that each user has a password (empty passwords don't count), and if you use the same username on both machines, set their passwords to the same thing. (Not terribly secure, but this is about getting them connected. You can secure them later.)
