A couple of things possible here...
a. What Win7 version are you using? The 64 bit version has been known to have some issues with drivers...
b. From where did you get drivers? Hopefully you aren't using one of those low-cost Chinese Prolific knock-offs - if you are, ...Prolific has put code into their load process to check if the chip set is counterfeit. If that's true, the drivers won't load....
c. What app are you using to connect (ARC996, FreeScan, ProScan, etc.)?
d. Did you go into Device Manager and see if something is grabbing the port you think you've allocated?
Mike