Unfortunately you have to wait for a deal on an OEM copy of XP, which on occasion appears for ~75-80, usually bundled with some crappy PCI card or a mouse or something. What I'd do is get a machine with XP on it and in a year or two or three when I wanted vista, buy a vista premium OEM license for $75.
Microsoft and its distributors are still well aware of the viability of XP and its value. In 3 years you might get a cheaper copy.
Problem with an OEM copy is you can install it on one machine and when that machine croaks or becomes obsolete, you cant transfer the license to a new machine. Its "bonded" to that original system and the MS droids will only let you activate it if you explain that you've changed a system board or something that triggered the activation requirement.
OEM Vista installs (what you get in the bargain sunday paper ad systems) usually come with an OEM install disk, you can buy one from the maker for ~$20, or you can make your own by burning a 'recovery disk' from some part of the crapware the manufacturer installs.
Mine for example allows for burning any number of recovery disks from a hidden recovery partition on the drive, or recovery directly from that partition via a key combination through the BIOS on boot time.
My HP machine on the other hand popped up on first boot and demanded that I make a set of recovery disks, then told me I wasnt allowed to make a second set and deleted its recovery partition.
So you COULD burn a set of vista OEM recovery disks, get an OEM version of XP when a sale item comes around (watch Fatwallet and put in a 'automated email reminder' if one comes up), install that and later "upgrade" to vista using the recovery disks.
I havent been as fortunate as W2R. Many of my XP apps that I bought over the years dont run on vista. In some cases I had to wait for a vista version and got that for free; in others I was expected to pay for a vista upgrade version; in others the manufacturer simply had no interest in making a vista version for the foreseeable future.
Ironically, the first app I installed once I had the system configured the way I liked and my data loaded was my backup/restore app (acronis trueimage). Turns out it wasnt vista compatible and didnt work. When I uninstalled it, vista rebooted and told me that it was broken and couldnt boot, but it'd fix itself. It then reported that there were no problems and it'd reboot, then that it was broken and couldnt boot, but it'd fix itself. After 5 iterations of that I quit.
When the manufacturer, who will remain nameless because it was Acer, didnt return my emails (they dont offer a telephone number for tech support, although I later found one and boy, were they sorry...) for several days and the recovery system didnt work and of course I hadnt made the recovery disks yet, I reinstalled vista from an "upgrade now!!!" disk that microsoft mistakenly put in the box with the laptop hoping I'd upgrade to vista "ultimate". I called MS and had them look up my OEM key that I'd already activated and reactivate this "retail" version for me. Not a lot of fun.
Later the system became so unstable that I decided to install XP, but since I decided I might want to run vista someday, I unhid the recovery partition, fixed it and broke the encryption on it, and was able to make a vista recovery disk. Vista upon booting again and seeing the recovery partition, hosed my primary partition and all my data...which fortunately I had a copy of.
Since restoring from the recovery disks I made, vista has sort of been okay. The video driver "blinks" now and then and thats allegedly a bug they're still figuring out; my touchpad periodically 'loses' its ability to do side scrolling; vista periodically decides to take 2-5 minutes to wake the laptop from a suspend...I think you're getting that its a bunch of little BS things that amount to growing pains, iffy drivers and application issues. Unfortunately, the driver and app people decided to wait until a few million people had vista before they'd rewrite their code.