Connecting to OS X server with Windows 7
You didn't mention what client or protocol/s you're using... could be you're talking about samba/cifs file sharing, and if so, there is an answer elsewhere on ask different (I'll try to find) that will fix your trouhle...

But I'll just throw this out there in case because I've done a lot of ARD/VNC and RDP the other way, and the issue that presents itself most (if definitely NOT a DNS issue) is that if the mac screen falls asleep, the client query for a desktop connection goes unanswered (very often), even if you check that box in the energy settings to wake for network access.

I have a couple solutions if that is what the problem is: 

The simplest solution is to set the screen to never sleep, or set a hot corner in the Screen Saver preferences panel, and leave your pointer in that corner when not in use. Its recommended you enable, in the Securtiy preference panel, check on the "require password after..." (to not leave your console completely inviting to any user).

The other solution I use is to ssh into the machine and "wake it up" with a bash script that uses osascript to simulate a key press. I believe the idea originates from a Mac Hints post from a while ago, and its posted here:

#!/bin/bash
# wake up the screen
# (will simulate pressing the cursor key)

osascript="/usr/bin/osascript"

osascript -e 'tell application "System Events" to key code 123'

exit


ssh in, copy into file called "wakes" on server; leave somewhere in your $PATH, chmod +x wakes, run the command, then immediately attempt connection with the desktop sharing client.