Slow internet and file access from Windows 2003 server, around 50 computers

I have around 50 computers and connected to MS File server 2003, sharing one harddisk for all the users for file access using MS word, XL, Adobe file , etc

10% of the computers are XP based and all others are Windows 7 64Bit with 12GB RAM i5 or i7 cpu, but all the users experiencing that the system is very very slow in opening a file which some time taking 4-5 minutes of few kb or 10mb file

using Level 24ort switch and cat 5 cable

please help