Friend,
A cisco VoIP phone is capable of setting the Precendence bits (COS) in its packets
Actually it sets the DSCP to EF and Cos to 5. The switch overrides these settings by default. You should configure your switch ports to trust the settings sent by the phone
to prioritise traffic for a particular PC, you can configure a Class/ mark the packets sent by the machine and set priority explictly
HTH, rate if it does
Narayan