Hopefully that config wasn't done by the installer without giving you options to change it. If it was, go get a better distro.
If you were given options but didn't know better, that's probably o.k. - you know better now ...
If you can boot that USB, best would be to completely install Linux there - including the loader to the USB MBR. That makes the systems completely independent. Someone here should be able to give you the (lilo) command - or just re-install. Then restore ntldr to the internal disk MBR (see below).
Another option is to have a small boot partition on the (internal) disk, and boot that. Probably will require a re-install in your case if you can provide the partition.
Next best is probably to restore the 'doze loader, and use it to chainload Linux. This takes some work to setup, and with lilo as your (Linux) loader is a significant PITA (that looks like a lilo error code).
In the interim, might be best to just restore the 'doze loader - boot the XP CD, bit the <r> key when it says so. That gets you into recovery console - from there run "fixmbr" (no quotes), and reboot.
You will have lost access to Mandriva - hopefully someone here can show you how to boot to recovery and rebuild lilo.