All of the OS's are fine, here is my thoughts though..
XP-I like this OS. You will have trouble finding it on a machine you just walk in and buy. You can order from Dell business and get it or it will most likely be a option on other machines. For personal use there is really no security risk. Its a great, quick OS.
Vista-Not bad, has a lot of stuff to help out someone that doesnt know much about computers, and focus' a lot more on security...again in a personal PC, it doesnt matter too much. This OS is good for someone that doesnt know much and like to change settings and such (built in UAC hlps you not F anything up, however the UAC is annoying to anyone that can use a computer lol and you will find it turned off on almost every PC).
7-Pretty good as well, I only played with this OS when it first came out and also some beta stuff. I havent really used it much day to day, but it feels more directed toward media and such. Cant really go wrong.
As far as mini laptops, there cool, but for tuning I found them hard to see the screen, and not able to see a full table when tuning. If you are just flashing files into the truck they work great, but if you actually have to do any real work on them they are tough to use IMO.
PCs at the store are going to be pushing windows 7, so if you want another OS you will prolly have to order the PC. I think Frys here has 1 PC with XP on it and its a POS, and 3 or 4 with vista on them. In your case, look for a PC in your price and has awesome specs, the OS wont really matter to you in your case...
... or from Dell to have a less "loaded down" laptop. :thumb:
Kidding right? A non loaded down laptop from Dell...Dell is notorious for loading a PC up with crap from other companies.
AOL comes to mind, does anyone even use AOL, lol?
If you buy from Dell, do not purchase from the home/personal side, go with small business...