windows 2003 server low performence

One of my customer complainting that server performence is slow.
thats as follows
Performance case ? well users not able to even execute the application

– Not able to open internet Explorer

– not able to open HPSD

– Disconnection to terminal server

i have checked that CPU usage average 10%
memory usage also below normal.
virtuval memory also ok
C drive free space also more than 10% free.

how to find and solve this issue. please any one can u suggest me greate thankfull.