Problems with connecting to Windows Machine

Hello all,

I have a Vista box on my network and I use the shared folder on the pc to dl backups for me. For some strange reason when I use Finder I can sometimes see my PC there..sometimes I cannot. And when I cannot, I have to restart my Mac so that my PC is visible again. Trying to get the PC visible is like playing Russian Roulette. This is infuriating as I cannot connect to my PC when I need it the most. Do you guys recommend any work around for this problem?

And my second problem is my Vista box detects my Mac as always but the only problem I cannot seem to connect my Mac. All the settings are good in the Mac to allow Windows sharing. However; when it gives me a choice to present my username and password it says its not valid. I try entering my Mac user and password and it still won't connect!

Thank you...