I"ve been using Windows XP x64 for about a year now. My previous OS of choice was Windows 2000 which I really loved, I swore to never upgrade to XP and in a way I succeeded because XP x64 is really just Windows Server 2003 x64 with another name. The reason I finally upgraded was so I could access all of my new system"s 8GB of RAM, and also because I was getting tired of writing Win2K fixes for games like Bioshock and Crysis. But from the very beginning of the transition I was disappointed; XP x64, when handling large files would slow down to a crawl.
I tried in vain to look for a solution, to give you an example: I"ve got a RAID5 array with 3 x 1TB hard drives and a lone Raptor 150GB for the OS. If I played back a 1080p trailer off Apple"s site stored to my hard drive using Quicktime Alternative the video would start to skip massively if at the same time I copied a large file from the RAID5 array to my Raptor disk, or extracted a file using WinRAR in the same manner.
Of course this is not the only problem, just an easy way to reproduce it. When I extracted large files I noticed that the Task Manager did not report memory being used under its graph, however the counter for available memory under "Physical Memory" would go down a whole lot, using as much as 4GB of RAM when copying files at least that large!