SVCHOST uses a large amount of memory for short periods of time

Every now and again my laptop starts to be choppy (from lagging mouse response to complete freeze).
If I manage to open the Task Manager I find SVCHOST using up all the memory it can get. After a minute or two it releases the memory and the computer retus to normal.

What is the cause and how can I prevent this from happening?

I'm ruing Windows 7 with 8 Gb RAM.