If you are running XP, get into the recovery console from the installation CD, and run 'fixmbr' an/or 'fixboot'.
If you can't boot off a floppy or a CDROM to get past this, than either you aren't useing bootable media, or your BIOS is set to look at your harddrive first, and dies.
I believe that the MBR points to the NTLDR (NT Loader) file, but if its not there, poof...
either way, booting off a 98 boot disk and running 'fdisk /mbr' will erase your MBR, and you can reinstall whatever you like. Installing Windows 98 alone will not do this, repartitioning will not do this, you need to erase or rebuild your MBR (which if you haven't been following, is the 'Master Boot Record', which is the first thing read off the harddrive, and tells the system where to look to boot the OS.