Rtman6, you should get the Shareware tool TaskInfo 2003 at
http://www.iarsn.com/taskinfo.html, which gives you more info on your running processes than the standard MS Task-Manager. For instance, it allows you to monitor the various threads within a process (here explorer.exe).
It is difficult to say in general what causes your CPU/Memory hog. Also, what memory were you referring to? Real memory or virtual memory? Note that if virtual memory increases, it doesn't necessarily mean that explorer is taking up more physical memory.
When you've installed TaskInfo, please post us all information of your running explorer.exe. For instance, on my computer (Windows XP SP1), which has been running without reboot for 4 days, 11 hours, explorer.exe shows:
InMem KB: 15.020
Private KB: 20.260
Total KB: 112.600
Handles: 403
Windows: 64
User Obj: 114
GDI Obj: 339
The actual physical memory used is InMem KB (here 15Mb).
Greets
Alex