Ok, so just because I can, and all of the below really happened to me this week I thought I'd share my story and see if anyone else can relate.

I've been running Vista Ultimate since about March of 2007. It took a while to get configured and running how I'd like, and that was no biggy. Compared to XP it didn't seem to hog resources so things were good. Up until about three weeks ago things were fine, I had nothing but good things to say about Vista.

But all of a sudden it caught the Microsoft itch. Unexpected slowdowns started presenting themself, with no viruses, malware, or any of that crap and no new software to report. Actually the speed was only really noticed in Maya, Auto Cad and Photoshop.
Vista seemed to be eating more memory than usual, a LOT more. There was a time in Vista's life when I could run Maya, Photoshop, firefox, and itunes all the the same time (dual monitors is great). And then I got to texturing some stuff in Maya, so I'd create a node and then try to apply a node to say the color of a blinn or lambert, or any attribute. Where a menu should pop up with a list of nodes, I got nothing, and on some good days maya crashed. One day this resolved itself and instead just lagged to death trying to load the menu. And then I soon realized the problem had copied itself to the render globals menu. It would crash maya or LAG to death. Again minor setback/annoyance when it was working. And then I'd try to open an image in photoshop. PS would now crash, or take forever due to heavy memory usage by Vista. All of my open programs combined for a total usage of about 1100 MB photoshop was asking for 200 which should be no problem - I have 3 gigs installed.

So I had to close maya, open photoshop. Close photoshop, open maya. just to make sure things were laying out correctly.

At that point I was ready to go back to XP, but then Microsoft said to me :Why not install service pack 1 for Vista?
I did some googling and found a lot of good talk about SP 1. So I downloaded the - not so surprising - 450 megs worth of it, installed it which again - not so surprisingly - took about an hour and like 8 restarts...?

After all was said and done it asked to restart one more time to make everything pretty. So I granted its final wish and hit the restart now button. I sat at my desk, watching in excitement to see my shiny new vista acting better than ever. Well 1 loading bar goes by, then two. Three, four, five, six and even seven go by. I figure maybe this is normal its trying to get everything all set in stone. Loading bar number 82 goes by (yes, I really wanted to get back to work and counting the load bars seemed to keep me distracted from the fact that windows was now into about 12 minutes of load time).

I wrote down the number of load bars and amount of time I sat there waiting and went to get some food. I came back about ten minutes later and Alas! The load bar was gone, I had progress to just a black screen... So not too long after that it booted into windows. My tablet driver had been screwed around with so I reinstalled that and restarted. This time windows would boot as quick as normal right? Wrong.

27 mind blowing minutes later I was closing tasks as soon as I could trying to quicken the load process just so I could shut the system down and install XP.

So here I stand on XP Pro woth service pack 3 which surprisingly was about 40 megs and took about 13 seconds to install and actually optimized the already awesome XP. For interests sake I did a fresh install of vista on another partition and well rendering results were astounding. A scene which rendered in about 6 and a half minutes on the fresh vista install rendered in only 4 and a half on XP.

As we speak I have autocad, itunes, firefox(obviously), thunderbird, msn, maya, photoshop, and after effects open and I sit here with 400 megs of ram to spare.

From my experience and the numbers I'd recommend XP to all. It was a great upgrade for me