I'm not sure I have much to offer but what I've seen. Reading/writing/editing ecm's and pcm's I only use WinXP 32bit and have never had an issue with TC on multiple laptops through the years. Shoot, I still have some software that I have to keep a Win98 laptop around for. I've never had an XP 64bit system. My programming laptops rarely ever see the internet and when I need to transfer files I use a thumb drive. Now that some software requires Win7 or newer(Holley for example), we have no choice but to have another laptop. I see nothing but problems with my Win7 64bit, screens flashing on and off for no reason, complete freeze of computer but my mouse still moves around and more crap than I care to remember.
Now that the later .net framework and Visual Studio stuff doesn't work on XP we have a lot of problems with older software and compatibility mode is useless.
The good news is refurbished XP laptops on ebay are very reasonable as long as you check the seller. Local computer refurbish guys on craigslist has worked for me too.

I wish I had a better answer for you.