Access your BIOS by hitting F2 repeatly when you boot the computer. Go to "SATA Drive Operation" under "Onboard Devices". Change the SATA Operation Mode to the opposite of w/e it is set to. This sounds crazy, I know. I have noticed that depending on the mode your using, the Vista DVD well only detect installations that were installed under it. Save changes and reboot to the Vista DVD. Now see if Vista can find the installation. If this does not solve the problem, change the setting back, backup important imformation if possible and follow the directons below to make the system the way it was the day you got it.
The following procedure well wipe all information on the hard drive that was not present on the system the day you recieved it, so make sure that your important files are backed up.
At the time of release of the Insprion 1501, Dell Pre-Loaded XP on that system up until the release of Vista. Prior to pre-installations of Vista or the Express Upgrade to Windows Vista Promotion on Dell Computers, you use the key-combination of CTRL+F11 during the Dell loading screen (Black screen w/ big Dell logo that appears the second you cut it on) to boot the factory image partition and restore the system.
Computers now that come with Vista pre-installed must be recovered in a different method described below:
Access the advanced options startup menu for Vista. (F8 immediately after the POST to access it.) Choose the System Restore option. There should be an option like "Restore to factory default settings" or "Dell PC restore". If it asks for an administrative login, provide one and continue with the proccess. The restore proccess should take only about 10 to 15 minutes and make it the way you had it when you first got it. If you do not see the option or cannot access the menu, you made need to reinstall the operating system from the install media. Call Dell support if your warranty hasn't expired if this is the case and you don't have the install media/experience w/ installing the operating system. (New systems recieve 1-yr standard phone based support from the time they're purchased).
In addition, messing around with the partitions on the drive may prevent you from using the system restore feature.