Rarewares has three different compiles of FLAC. They have the bundle, Generic, and P4 only.

I understand what the bundle is, but what's the difference between the Generic & P4 Only compile?

Could guess that the 'generic' is for all kinds of CPUs and the 'P4 only' was optimized for P4 CPUs. So if you use the P4 version on a non P4 CPU it will probably not work well or even crash.

My guess is that P4 is equal to "CPU that supports SSE2" so not for AthlonXP but Athlon64. In fact I'm not having problems on my A64.

weaker

The 'bundle' is a plain vanilla VC6 compile that should run on just about anything. The 'Generic' ICL compile is similar except that it is compiled with the Intel compiler and may run faster, but may be less compatible, and the P4 compile, as already mentioned, requires P4 compatibility and should be faster on such systems, although these are only compiler optimisations, not custom code.