When I open c: drive it asks for open with.

My system configuration is
865 chipset mother board
80GB HDD
1GB RAM
WIN-XP operating system with sp2
0ffice-2003
Sql2005
Ez Antivirus softwares with latest virus definitions

Is there any solution without formatting the HDD.:)

Correct me if I'm wrong, but if you have the CD with the original Windows XP Operating System on it, and insert it when you turn your PC on, it usually goes to setup and then asks if you want to install or repair... If this is the case, then try the repair.

Another suggestions, is that you use the Boot Disk (if you still have floppy drive) that you should have created when right after the Operating System installation. It is always recommended to created a boot disk, boot CD, or a complete Recovery CD set to ensure that you don't lose critical data or files get currupted.

Also, if you can get in, and can go to the 'run' feature, try typing in cmd.. should get you to DOS command line..
At C: type in dir and see if you still have a Windows directory.. if not OOPS.
Another try would be at c: type in chkdsk/f and let it check your disk - if that is still possible.
Our computer experts should be on soon, and will probably give you more and better hints..
Hope this helped. Good luck.
On the brighter side.. Happy Holidays!