CPU use up by SQL with unknown process

I use SQL server 7.0 ent. version as the database.
My server is a 800MHz single CPU with 4G RAM.
SQL monitor shows 20~30% CPU usage in normal situation. Sometime it shows 100% usage for a very short moment but it is still normal.

However, userscomplain the system is very slow and I found the CPU usage is 100%.
I use sp_who2 but find no process using large amount of CPU.
I use the NT’s task manager, it shows the SQL server use up the CPU.
My question are:-
what is the SQL server doing and use up the CPU?
How can I check it out and prevent it?

Many thanks!