It works with windows 7 64 bit, I'll show you how I got mine to work.
First off follow this link to get the 64 bit (or 32 bit drivers) for the eris and windows 7.
1. Workaround for HTC Hero Sync Problem in Windows 7 My Digital Life
unpack this file somewhere in a temp folder or something where you know where it is.
2. if you have installed the HTC sync package, uninstall it for now.
3. go to settings>applications>development and switch on USB debugging, then plug your Eris up to your computer.
4. Go into your device manager and you'll see an ! ADP device or something like that, right click that and click install driver then navigate to your folder where you unpacked the driver and install it. the error in device manager should go away and you should now have a new device setting in device manager called "android USB device, open that and it should say something like "my HTC". You now have the correct drivers set up.
5. Download the HTC sync program and save it somewhere. Right click it and set compatibility to Vista SP2. after you do that right click and select install (or run, i forget) as administrator. You will get some error messages, "ok" through them and the actual program will install (the error message pertain to the program trying to install it's on drivers. i think i remember). In any case the HTC should now see the phone and you can setup what you would like to sync and start syncing your phone.
I will try to find the other URL that explains this as well, but it works for me now. This is actually a work around for the HTC Hero, but it works just the same for the Eris.
PS first time post, couple months of lurking, hope it works for you