i changed the graphical card and trying to install Centos 5.8,but am unable to boot my OS using DVD,i am getting the following error.
Am requesting u guys to help me to sort out this error.
Reboot and select proper boot device
or Insert Boot Media in selected Boot device and press a key

Just to clarify, you had windows 98 on this machine and are now trying to install CentOS, correct?
Did you format the windows 98 partition?
The obvious questions first, you do have a DVD drive? you have that drive set to first boot priority in the BIOS.
How old is the machine? Post some hardware information.

Thank you,No i just used windows 98 CD instead of boot ghost to delete the linux partitions tat's it

Machine is with i5 processor,4gb RAM,with ZOTAC VGA card

I have few question for you, understanding that we will be able to help you.

1- So you got a new video card and so you want also a new system(CentOS 5.8)?

2- Is CentOS installed? If not, then which system are you running?
I see RedHat icon under your nick, is that a different machine?

3- Are you trying to boot a system within your CD/DVD drive, which disc are you using? If not this then:
Is the machine asking for a disk when you start it? Is the machine booting at all?

Please note this questions will help us to help you, we do need information to understand the problem,
be very clear in you statements, that way we will have a clear sight about the problem.

Regards