I replaced a motherboard in dell dim...

Put your Vista CD back in the machine and hit any key to boot off the cd. After windows loads it's files it will ask you what you want to do. Choose repair, it will search for windows installations and list them, there should be only one listed. Highlight it and choose repair. Windows will search the installation for problems and attempt to repair them. This make take a minute or an hour depending on the problem. Once it is finished it will tell you to reboot. Take the disc out and reboot the machine. It may be wide when it reboots to try to get it to boot to safe mode first to see if it actually will fire Vista up, if it does great, reboot again and let it go normally.