On Windows7, right click on the drive, click Share With > Advanced, then click the Advanced Sharing button, tick "share this folder" and give it a name"
Then on your second machine, click start > run > and type in the name of the pc followed by the share name, for example:
\\Computer1\sharename\ ... it'll either show you the share, or usually prompt for login credentials, so you'll need to enter some valid credentials from PC1, although it's probably best you setup a local account on PC1 that you login with on PC2 so its passive
Once you can see the share as normal, map a network drive to it