Sound like your MBR [master boot record] has become corrupted.
For further info
Look at: -
http://www.pcguide.com/ts/x/comp/hdd/fail_Failure.htm
Recommend in future that you obtain download & build the following: -
Bart PE Rescue CD
I got bored reading all the other replies, but I did see that one of the posts mentioned PE Builder or Bart PE.
Bart PE is the basis for a number of simply amazing Rescue CDs / DVDs. These are mostly free and highly customisable.
I recommend that you visit the following websites
Dos Based
http://www.ultimatebootcd.com/
Many Dos Based utilities, a must-have for any IBM compatible PC / Laptop [it's Linux actually, but you'd never know]. A straight download.
Bart PE Based
A must-have for any IBM compatible PC / Laptop. Because these are Bart PE based these offer NTFS read/write, but must not be used on Windows NT without SP 4! Fine on Win NT SP 4, 2000, XP, 2003.
For Dell, you may have to make a modified version, instructions on the fora.
http://www.ubcd4win.com/ UBCD4Win
http://www.nu2.nu/pebuilder/ Bart PE
http://www.reatogo.de/ Reatogo
These offer a wide range of utilities to diagnose / fix your PC / rescue data / remove viruses, spyware and other nasties.
What's the downside, well the Bart PE Based versions are available only as a kit. The reason for the kit is Micro$oft! Therefore you make it from your installation CD and its use is covered by your Micro$oft license. If you don't have a CD, it is possible to build it from the Windows files on your HDD.
Linux Based
http://www.knopper.net/knoppix/index-en.html
2 versions CD and DVD
Tends to be experimental ? At your own risk!
http://www.ubcd4win.com/ Ultimate Boot CD for Windows
You will also find help on the fora regarding assistance in booting from CD/DVD.