unbrowseable comptuer

Hello i have a couple of computers that have the following problem,

when i try to browse them through the network using the UNC, it gives me and error. I try to ping it, and it returns a reply no problem. If i reboot the machines i can access them through the UNC and then it stops again. i checked port 139 with microsfots portqry tool and it says filtered. any ideas?

thanks,