Ok here's the trick to fix TI-Nspire compatibilities issues with Windows 7:
First of all, make sure you downloaded TI-Nspire Computer Link Software from
http://education.ti.com/educationportal/downloadcenter/SoftwareDetail.do?website=US&tabId=1&appId=6660 . The install from the CD that comes with the Nspire will not work. Also, DO NOT PLUG YOUR NSPIRE BEFORE INSTALLING THE SOFTWARE!!! Once you got TI-NspireLink.exe, follow the instructions below:
1: disconnect your computer from the Internet. This is VERY important so Windows won't try to find the drivers online.
2: Run TI-NspireLink.exe
3: On the Windows compatibility warning, click "Continue" anyway.
4: Install the software normally.
5: Once the install is done, connect the TI-Nspire with your computer.
6: Wait until the automatic driver detection is done. It will fail...
7: Go in Start->Control Panel->Device Manager
8: Right click on TI-nSpire Handheld then click Properties, then go to Driver->Reinstall Driver->Choose a driver on the hard drive
9: Go in the TI-Nspire Computer Link Software install directory and go to drivers\Win 64-bit Vista\ then click OK
10: Select the only file then click OK until it's done.
Hopefully it should fix your issue, altough the first time I think I did it wrong since it took me 3 tries until it works.
Thanks to Levak from TI-BANK for this trick.
http://tibank.forumactif.com/problemes-divers-f9/windows-7-64bits-et-nspire-t4830.htm