How To Modify The Boot Loader On Drive

I installed a beta of Vista just to try it out. I used Partition Magic to create a second partition to run it on. I installed it with no problems and was able to boot between Vista and XP when I needed to switch. When I was done with Vista, I didn't uninstall using Vista's uninstaller. Instead I used Partition Magic to reformat the partition. I've installed Ubuntu on the partition that Vista was on and I like that pretty well. Problem is, when I try to boot into XP, it still thinks that Vista is installed on there and goes through Vista's dual boot prog. When I select XP it loads normally until it gets to the part where it should run autochk. It crashes saying it can't find it (it must be looking for Vista's copy since XP still has its) and I can't do anything except turn the machine off. Point of the story: How do I modify the boot loader on my XP drive? I can still access the infor mation from Ubuntu so if I have to edit something I can.