Wow, just...wow. Vista and XP can share the same Hard Drive on separate partitions, yes. However...they will not be very amicable towards each other. It's best if they live of separate Drives altogether.
Ok, to the OP, my advice (and the advice of several others on these Forums) will be to completely ignore everything PC has told you so far. He's turning a super-simple procedure into screenshots galore, Beta test RCI this, and as seen here that. If you want a few good chuckles, just click the second link in my sig - very good stuff!
You installed Vista first, XP second, correct? Now that XP is loaded, you can't boot into Vista - ok, gotcha

There are two ways to fix this. The first (and simplest if command lines make you nervous (and they do to a lot of folks, so no worries if they do!) is to install EasyBCD, as mentioned a few times before. Follow the on-screen instructions, and you're most-like done. Just like Staples...that was easy; and it only took two sentences to get out, as opposed to several multi-paragraph Posts. But, if BCD is being funny, or if you want to play with a command line...
...here's how ya do it. First a recap, Vista was installed first, XP second - assuming the XP Install is finished and you boot right into it upon startup, put in your Vista DVD and restart. Get to the Installation screen (but don't click Install!); instead click "Repair" then "Next" (Vista will be the only thing you see here - don't worry, XP is still there). Choose the option to open a Command Line and, once it's opened, type
Bootrec.exe /fixMBR and press Enter, then
Bootrec.exe /fixBoot and press Enter. Exit from that area and restart your system. It will auto-boot into Vista this time, but once again, all is good. Once in Vista, open another Command Line, this time as Admin (right-click, run as Admin) and type
bcdedit.exe and type the following bold commands below (***where "X" in the first line is whatever partition XP is installed on as per Vista's say-so - if you didn't partition the Drive you have Vista on, and you only have the two Drives, this will most-likely be D) (***if X will in fact be D, feel free to copy and paste this exactly, one line at a time, and press Enter after each one is pasted into the Command Terminal)
bcdedit –set {ntldr} device partition=D:
bcdedit –set {ntldr} path \ntldr
bcdedit –displayorder {ntldr} –addlast
bcdedit -set {ntldr} description "Microsoft Windows XP"
Once that's done, you can exit out of it and Restart. Grab a piece of cake and eat it because you're done!