Windows 2003 Server Performance Reduction

Hey guys I have a major problem I have a Windows 2003 Standard server which has all of a sudden gone from rabbit to hare. The server hosts my file services, & a few SQL Dbases. I have all the recent MS SPs & patches installed etc. In terms of specs it runs on two 3 GHz Xeons, 4 Gigs of RAM 1 Terabyte of harddisk space. Want i want to know is what are the possible causes of the reduction, where should i being to look, let me also mention that this seems to happen at set times, lets say 11am and by 3pm. at which times i have to restart the server.