If your computer was running slow before this happened...it could be malware related.
If you're getting an error message that there is no boot sector on the internal drive, then being able to boot into Windows may be a problem...since there is no boot sector.
Have you tried booting into Safe Mode? Reboot your computer and before it gets to the Windows boot screen...continually press the F8 key on your keyboard to boot into Safe Mode menu. Once there, select Safe Mode and press the Enter key on your keyboard. Let us know if you were able to do this, successfully.
If not, then there's a pretty good chance the Windows installation is corrupt and needs to be repaired...and/or the hard drive needs to be re-partitioned, formatted and the operating system needs to be re-installed.
Please note: Before you do the above...you need to be aware that re-partitioning and reformatting your hard drive will result in the loss of all data on that drive. You will NOT be able to recover any data once you do this.
Also, do you have the system recovery CDs that came with this Dell computer?
Please keep us posted.