You will know by either editing with NotePad or using the boot.ini tab found in the msconfig utility which one is for the first partition. A quick fix will see the one installation on the first primary takeover. You can simply copy the info seen in the screen shot here in order to verify what you end up with in the file there.
All you have to do in NotePad is simply use the backward key once you are at the end of the partition(2) entry and wipe that one leaving the default for the 1st intact. When going to save the file save it as all files not a doc or txt type in order to see the corrected version overwrite the original or simply use the msconfig.
This is why right clicking on the file first to uncheck the read only or hidden boxes and clicking the apply button has to be looked to make sure you will be able to overwrite it when saving the correction with NotePad. One alternate method for seeing this done is simply booting to the recovery console and using the "Fixboot" and "Fixmbr" commands there to see a new master boot record created which will correct any goof in the boot.ini you make there.
That's an even easier method if you are nervious about editing the file. I thought I would add this in as well as the 3rd and easier method. With Vista there is no recovery console available since MS threw in an automatic tool to fix startup problems found on the installation disk.
(too bad a Europeon court just ruled against them for having WMP included in Windows.
Microsoft hasn't responded yet.)