There could be many reasons why explorer.exe consumes high amount or memory. This particular issue has been reported on Windows Vista and Windows Small Business Server 2008 machines and is caused by Windows Search functionality.

If you click on START and type anything in the “Start Search” box, explorer.exe RAM usage starts increasing. This doesn’t stop even after Start menu is closed and eventually explorer.exe consumes pretty much all remaining RAM. Some users also report high CPU usage. Killing explorer.exe and starting it again fixes the problem until the next time search box is used.

To fix this:

Right click on Start > Properties > Start Menu – Customize.
Scroll down to Search files and select Don’t search for files

Windows Small Business Server 2008 (SBS 2008)
Windows Vista