Invalid System Disk???

Greetings & Salutations,
When I boot up my unit, I get a DOS error message:
"Invalid System Disk - Replace the disk, and then press any key".
There are not any CDs/DVDs in the drive bays.So I hit a key and it continues as normal
Any Ideas as to why this is happening?
Thanks in advance to all who help!!!

Jack

 

Just wondering.....in your BIOS is the bootup sequence correct?

 

FYI:

http://support.microsoft.com/kb/82923/EN-US/

 

Thanks Managed!
I don't know how to do that.
Is it a simple proceedure?

Jack

 

Thanks DCD!
How do I check that?

Jack

 

Reboot. Keep hitting F2 until you get to your bios and search for boot sequence and change it around. (Without going to my bios it's under Boot or something). Just switch around to 1st Boot - CD-Rom and make the one now as 2nd Boot.