CPU Usage reaches 100%

CPU Usage reaches 100%,

When I connect to the internet, I go to Windows Task Manager, the CPU Usage reaches 100% after 2, 3 hours.

My system: OS: win pro 2000, SQL 2000, …

The sqlservr.exe process is highest (around 95% all the time).
I have to reboot my desktop and it becomes normal again (CPU Usage is around 4% to 10%)

How could you explain this?

Thanks,
Tuan Do