|Apparently your model is a desktop computer|
Your hard drive is definately failing. It cannot be fixed by anything you can do yourself, unless you're a data recovery expert, or want to take a stab at trying to be one - it will only get worse.
You can get the drive replaced on warranty (5 years for most more recent Seagate desktop drives; 3 years for other models and older ones) but ....
- you may not want to bother - it will take some time to replace the drive - you can easily get another 500gb (or larger) drive locally right away, for a lot more cheaply than when the computer was new.
- you must go to the Seagate web site, and follow links to find out how to RMA the drive - you must supply the code(s) you are getting in the SeaTools tests, follow the shipping packaging guidelines, and ship the drive at your expense (the slower the method, the cheaper) to some location - they will pay for the faster shipping back to you, replace the drive, probably with exactly the same model, and the warranty is then only the remainder of the original warranty, although the replacement will probably last at least 5 years.
- OR - you can have them ship a replacement drive to you, probably exactly the same model, and you ship back your ailing drive to them after you get the replacement, within a specified period, but in that case they ship it to you a fast but more expensive way (so you pay shipping both ways) and you must supply credit card info to them - the advantage of that is you get the drive in a short time, you can possibly copy data you want to save from one drive to the other on your own computer before you send your old drive back, and the packaging they provide is of course already appropriate (ship it back as cheaply as possible if you like, as long as it will get there in time).
If you have data on it you don't want to lose (e.g. email, personal documents, etc.) that can't be reloaded from CDs or DVDs you have or from downloads from the web, etc. (downloads you paid for can often be downloaded and activated again - see the appropriate web site)....
- if you can still access the drive in Windows, copy the data you need to save to CDs or DVDs or flash drives or to a second hard drive if you have one.
- if you can't access Windows and that's the only hard drive on this computer, you need to connect it to another computer as slave or master but don't boot from it, and copy what you can elsewhere, if you can.
- if you can't access the drive at all, you can take it to a local data recovery expert (e.g. some computer repair places also do that), or send it to one on the web, but that can be expensive - although, most of them will not charge you or only charge a minimal fee if they can't recover any data - but usually they can.
- if you're handy and very careful, you could do part of what a data recovery expert does yourself - you could buy exactly the same model of drive and try swapping the logic boards(that's usually where the problem is), or if that doesn't help, very carefully transfer the disk(s) (platter[s]) inside the drive to the working drive.
Your Dell model may have come with a Recovery DVD or DVDs, that if you follow a Recovery procedure, can reload all the data that was on C orginally when you got the computer, but it (they) may require that the second partition that is always on the original hard drive on a brand name system has data on it that is intact and undamaged - so if that data is not intact and undamaged, or if it's not there, it(they) may be useless regarding loading a new blank drive.
There is almost always a program on brand name computers that was put there by the brand with which you can make a Recovery disk or a Recovery disk set, while Windows is still working well enough for you to do that. If the data on the second partition is damaged, you would probably need the Recovery disk set to load a blank hard drive.
If you did not make a Recovery disk set, you MAY be able to order one for your exact model on the Dell web site - if it's available, it's probably cheaper than even a OEM Vista Basic CD.
If you didn't make and can't get a Recovery disk set for your model, your OEM Vista license allows that you can legally use a bootable COPY of someone elses's OEM (or retail?) Vista Ultimate DVD (full version) along with the Vista Product Key that is on the official Microsoft label on the outside of your case somewhere, but that entails more additional things you probably need to do (e.g. load main chipset drivers, and/or load specific device drivers found on the brand name's web site in the downloads for your model) for some things after Setup has finished than when using the Recovery disk set.