That's the reason you can't enter Windows Vista. The installation was done while the drive was in IDE mode. If you are simply doing a clean install of Windows 7, just drop the Win7 DVD in the drive and boot to that disk.
If you are trying to keep most of your files from the Vista installation and do an upgrade installation, you will have to switch back to IDE mode. Then after Win7 is installed in IDE mode...
Follow this fix to get the system back to AHCI mode:
http://support.microsoft.com/kb/922976
I've used MS's fixes for other things and they always worked well. But I have not used one for this problem, so I can't vouch for this fix.
There are several DIY fixes if you search for something like "Enabling AHCI in Win 7 after installation".