I had this problem back in 1.7.2, and when I updated to 1.7.4 it seemed to be fixed for a while, but it has started happening again.
As I play, performance gets worse and worse, and usually a second javaw.exe process starts taking up more and more of the CPU, which, when added to the original process, usually consumes close to 100% of the cycles eventually. This makes minecraft pretty much unplayable, of course.
I thought maybe it was because I had both 32 and 64-bit versions of Java installed, but I uninstalled 32-bit Java and the problem still occurred.
Back before the 1.7.4 update I posted on the minecraft forums about this problem, and no one there could figure out the problem.
Before I switched from XP to Windows 7 I never had this problem, even in 1.7.2.