This is one of those times where the XP rescue system comes in handy

I don't have Vista so I can't tell you exactly what to do there, but I know XP has a rescue system on the CD, so boot from one of them and repair it. You should be fine, the NTLDR is "NT Loader". It's kind of like Lilo or GRUB on Linux, except for Windows, and just loads OSs.

A quickie google on "NTLDR fix" should point you in the right direction.

And don't worry, I've screwed up MBRs and partitions far worse than this. Heh, had a Redhat install fail WHILE doing the lovely task of writing data to the MBR which left the machine in an "I'm not booting" state.

Luckily I sat down, installed a new copy of a different distro over it and it overwrote the MBR.

For Windows 98 SE, I once was going to tri-boot. I had it set up and was installing the third OS (FreeBSD) and it failed. Now normally that isn't a problem but at the time I couldn't get Linux to use my modem (This was back in early 2000 before it actually could recognize hardware like that by itself, so I was pretty well stuck), when I rebooted, FreeBSD would load but since only a base install was there, NO apps would load, and with Linux I could boot into it, but couldn't get to Windows as for some reason something happened during the install and it just woul not load.

So my boot screen showed Windows 98 SE, Linux, and FreeBSD, but only Linux booted into a useable state.

To make matters more interesting, I had JUST whiped my back ups, thinking I would do a fresh back up afterwards. (It was late, I was tired, wasn't thinking to good, so obviously that really bad idea of whiping back ups before doing this was stupid as it is, but thinking somehow nothing could go wrong and I'd do another back up later was somehow going to work was plain dumb).

I lost a lot of data but fixed it with Fdisk on a floppy, and just deleted every partition.

Another time, I screwed up the MBR so bad I had to edit it in hex.... I don't remember HOW I managed to do it, but the thing booted afterwards.