Did you do a fresh install of Windows? You say the HDD is new, did you clone the drive from the old HDD? This info will help us narrow down an answer for you. If you have a Windows installer disk or made a recovery disk, you can use them to check and repair your install if possible. If you have a Dell, they may not have given you a recovery disk, as most OEM's use a hidden recovery partition on the original HDD. You can see if Dell will issue a disk to you, or you can go online to digital river and download one of the Microsoft disk ISO files and make your own disk. This will require access to a working machine, and you may need your license key, which should have been affixed to your PC.
Im not sure what version of Windows you were...