Vista - No login screen, dual-boot problem

Dear All,
I installed linux (centOS) and Vista in my laptop. My Vista and linux systems were running fine for the last 2 months. But suddenly as I start my laptop I saw thousands of exclamation sign and then if I choose Vista, I see 'Microsoft Corporation' and the progress bar.....but my login screen does not
come! If I start linux, I get an error message about X server; that is can not start. As a result I do not get any graphical interface; I get only a command-line. As a result I
can not use any system! How do I check if my Graphic card is OK?

Please help.
Thank you for your kind support.

Sincerely,
sonali

Sorry, but CentOS is one of the few Linux distros that I don't have experience with ... so my suggestions are rather generic.

While it COULD be the graphics card, since you got a login screen with a progress bar on Vista, that tends to indicate that the card is working properly.

Failure to boot on both OS's is more an indication of possible hard drive failure that caused filesystem corruption.

If you have a Vista DVD, I would boot to a command line in that and run "chkdsk" to look for NTFS filesystem errors.

If you have a "LiveCD" for CentOS, I would also boot to that and run "fsck" to check that filesystem for errors.