Ever wondered why the svchost.exe process seems to be consuming an inordinate amount of RAM and/or virtual memory? Do a Google search and you'll see that many other people are wondering why this memory-hogging occurs.
Wonder no more. The offending process is Automatic Updates. You can see the processes hosted by each svchost.exe by using Process Explorer by SysInternals. Just select the svchost.exe in question and choose 'properties'.
On our corporate computers, where access to Automatic Updates is blocked by a global policy setting (as is access to Windows Update itself), the Automatic Updates process gradually consumes more and more memory. After roughly three weeks, the svchost.exe in question was using about 200 MB of RAM and over 400 MB of virtual memory.
The solution? Disable the Automatic Updates service. Right-click on My Computer, select Manage, then select the Services tab and change the Startup Type for Automatic Updates to "disabled".
If you never use Automatic Updates, or Group Policy Settings do not permit usage of it, then there's no point running the service anyway.