The one thing that people forget when networking an Apple computer to a Windows network is making sure the workgroup name is the same as the Windows workgroup name.
Windows 7 and Windows Vista usually use the Workgroup Name
So how do you do this on your Apple Computer?
Open Network in the Settings panel
Click on the Advanced tab
Open the Wins tab
Type workgroup in the workgroup name
Then press Apply as it returns to the Network panel
Also make sure you have the correct settings to share files