MS SQL7.0 Running Too Slowly

Hi, I’ve a MS SQL 7.0 (SP2)running under NT 4.0 (SP6a), with 2 X PIII 800 processors and 2 GB of physical RAM and 2 GB of page file. We are facing a tremendous slowness at this moment. No changes has been done in the server or applications. Data processing (similar # of records) has been slow down from 3 hours to 6 hours, sometimes 9 hours ! Why ? When this happen,the memory utilization in the task manager shows only 50% of the usage, CPU on 20%, but the performance monitor for SQL shows 97% utilization ! Why ? Also, sqlservr.exe in the process task list shows usage of almost 2GB of memory utilization. Why ? How ?….
Please…..help…..