BSOD When i logon

Hi, i have win2k advanced server installed on my compaq ML370 server.I have standalone servers not clusters. on this Server AD is is installed as well as Exchange 2k for the corporate mail system.Of late when i restarted this machine all services were successfull until i came to the log on prompt,once i log on , i get the BSOD and i have to restart the machine in ad recovery mode and all diagonistic modes that are available but alas it fails once i log on using any account. IF i dont log on themachine works ok and the mail system is fine. This has been on for about a month now . How can i resolve this?