Vista - connecting vista to imac

so i have a vista home premium 32bit and an imac running os x 10.5.6.
when i first purchsed the imac i could connect from my vista to the imac, accessing the shared files. then one day i couldnt connect. i can still mount the vista on the imac and access the shared files, but i cannot get onto the mac from my vista.
i was finding the imac in the network map, and just not being able to log onto it cause it said the password or username are incorrect, but their not...
and now i cant even find the imac in the network, its just not there and ive tried typing in the ip address to find it, but that wont work either

ive seen in various forums to type in secpol.msc in run, but this does nothing, says there is no file or program, therefore i cant change the permissions or whatever the rest tells you to do.

i am honestly ready to smash both the computers and i would really like some help.