Invalid System Disk error

[#1]

Booted up my PC this morning, only to be met by a black screen and the error message "Invalid system disk error; replace disk and try again." I tried booting up a couple of times - same thing. No floppy disk in drive - so can't be that. Then I disconnected a card reader (USB) that had a new compact flash card in it and lo and behold, the PC booted up fine.
My question is - could the non start error have been due to this flash disk, or is something more sinister likely to be happening? How can I find out?
I have AVG running on the system and Zone Alarm. Windows XP. Thanks for any ideas.

[#2]

Most probably.
You could check your BIOS and ensure that the boot order puts your HDD ahead of any other devices you don't want to boot from.

[#3]

As James has suggested, it may be a BIOS issue.
When your computer boots up, it initially runs the BIOS, which does various low-level tasks.  One of these tasks is to enumerate all the fixed disks on the system.
Different BIOSes enumerate different types of drives in different orders, so they might be 'seeing' your card readers as a bootable 'floppy', or recognising them as removable media.
Within the BIOS, there should be the option to select a 'Boot order' or 'Boot Priority' which should allow you to prioritise which devices are checked first.
Personally, I would set this to be just the Primary HDD and turn all other boot options off (it's slightly safer from a virus perspective and if you do subsequently need to boot off another device you can re-enable it)
B.

[#4]

Quote from: newinvention

I tried booting up a couple of times - same thing. No floppy disk in drive - so can't be that. Then I disconnected a card reader (USB) that had a new compact flash card in it and lo and behold, the PC booted up fine.

You may wanna do a simple test and plug the card reader with the new compact flash card back in..  reboot and see if you get the error again..  If so that pretty much confirms the bios issue...
regards