does enterprise server 5.1 mp4 support multiple processors in Windows 2003

my issue is this. I have a 2 proc dell 2850 during backups 1 CPU is peaked
at 99% and the other processor is around 5%.

i tried switching my box to enable hyperthreading. now when i backup 1 of
the 4 processors is 99% and the other 3 are only at 5-10% tops.

is this an issue with 5.1? i can't find anything on processor support for
5.1 but 6.0 states it supports multiple processors