You unplugged the 320 *after* you installed on the 500, right? It's possible that the boot instructions were (attempted to be) installed on the 320.
Now try to install again on the 500, this time without the 320 plugged in.
Hmm, maybe it won't be necessary, you can do a Startup Repair by booting with the Windows 7 installer DVD:http://windows.microsoft.com/en-US/windows7/products/features/startup-repair
Does the 500 show A-OK in BIOS?