Problem pinging computer (XP family / pro)

I'm having a problem with a network I'm setuping at home... I've been trying to figure it out and search for assistance for the past few weeks.

Basically, I have 2 comps; both are connected on a router, which is on a DSL connection.
comp#1 has XP pro
comp#2 has XP family

Both computers are able to connect to the internet with no problem, however, they can't see eachother in the workgroup... (sometimes comp#2 sees comp#1, but cannot access it, sometimes it just doesn't see it at all) ... I tried the 'ping' command both ways, and I figured out that comp#2 couldn't ping comp#1 (even tho comp#1 can ping comp#2 no problem)

Also, my friend came here yesterday and brought his laptop with XP Family and he connected on the router too, he had no problem sharing with comp#2, but couldn't ping comp#1 either...

There is no firewall activated

So I'm guessing the problem is on comp#1 since the other machines can't ping it ; but what could cause it to be unable to respond to pings???

(For more info about comp's config, network setup and past problems/diagnostics/solutions, check this thread from another forum --- this forum seems down at the time I'm posting this, hope it will be back up when you read this :P)

Thanks