Vista and XP use different boot managers (bootstrappers). Win 7 RC is compatible with Vista, but is just as touchy as Vista when XP is involved.
Try this site first and read the option from the pull-down that fits your circumstance:
Link Removed - Invalid URL
Then also go to the EasyBCD site and read their FAQ about XP. Between the two sources, I am certain you will sort it out
I am curious, though. I am not criticizing, just asking a question - Why are you going from XP to 64-bit Win 7 RC instead of 32-bit? Anyway, let us know how it goes. There may be others in your same boat. I know from experience that while XP and Vista/Win 7 can coexist, it takes a little tweaking on our part. I have Vista 64-bit and Win 7 RC 64-bit on one machine (Gateway DX4200-09), and XP and Win 7 RC 32-bit on another (Gateway 550GR). They both dual boot without a problem, but I had to use EasyBCD to get there with the XP system. I also have an HTPC I built from scratch on which I am currently using Vista 64-bit, and I am looking forward to switching to Win 7 upon its final release.
P.S. I know it says Vista above, but for all intents and purposes, Win 7 is Vista on steroids.