I use Windows 7 Home Premium 64-bit, and have no problem tethering my 5D MARK II to LR3.3 (although I have to wait for the BUSY message to disappear after a few seconds on my camera). Have you tried tethering with Canon's EOS Utility?
Are you using a 5D Mark I or Mark II? The Mark I is only supported on 32 Bit O/S, with the driver from Canon. Have you been able to tether with EOS Utility on your Win7?
The solution to this problem (tethering a Canon 5D classic) to a Windows 7 Professional 32bit laptop takes a little doing but it can be done.
Install Virtual PC and Windows XP mode for Windows 7.(http://www.microsoft.com/windows/virtual-pc/support/default.aspx)
Once you have done this and rebooted, set the compatibility mode of Lightroom to WinXP Service Pack 3.
Next get your EOS Solution Disk (ver 12.1 or so) put it in your drive but don't run it yet. Once it's in the drive open up the Computer window and right click on the drive. Explode the disk and find the installer program. Set the compatibility of the installer program to WinXP and then you can run the installer to install the 5D driver.
Now when you start LR it will recognize your 5D classic and life has been restored to normalcy.