If you installed Windows Vista okay the first time the hard drive should be okay however I would advise running the Dell diagnostics just to be sure. To do this follow the instructions in page 405 of this document and run any extended tests. [URL]  What Video Card do you have? Have you tried removing the video card and running using the onboard video? Did you install Service Pack 1 before the system drivers?
A friend had a Dell Dimension E521 that wouldn't boot - had the system\hal.dll file missing. As there was nothing on this computer he needed, I rebuilt it installing a totally fresh copy of Windows XP, used the enclosed upgrade disk to upgrade to Vista and made sure everything else in the original package was installed. I left it installing Windows updates overnight and in the morning, after a restart it indicated there was a update available. I updated that and restarted and it indicated there were four more updates which from memory where about the video card. I updated those but now it won't boot up. I goes through the boot process but as soon as it comes to the black screen with Microsoft Corporation and the (green?) bar it just starts to progress a tiny amount and then restarts. It comes up with the Windows start option screen and I have tried Safe Boot and Last Known Good Configuration to no avail. I have tried the original XP disk to do a repair but this doesn't seem to work with Vista??? How can I get a working computer back or do I have to reinstall again? Thanks for any help anyone can offer.
Dell Dimension wont boot into Vista after upgrade