Can't Browse the Network?

I used to be able to, but now I can't. All the Machines are either Windows XP or Windows 2000. When I double click on My Network Places and hit View Workgroup Computers and I get the error, the list of servers for this group is unavailable. What happened? My brother's PC was infested with spyware... so I reformatted. My theory was that maybe somebody hacked it, and screwed around with my router settings. I checked everything and it seems to be OK. I can ping each machine. If I search for the computers, I can find them and access the shares. Just not browse. I gave it an hour at one point (while I was doing wash) and it still couldn't browse.