Windows Explorer and IE 7 very slow to respond

Hello,

I have this very frustrating problem with both Windows Explorer and Internet
Explorer 7. I'm currently running Windows Vista Ultimate on a Dell Precision
380 Workstation with 4 gigs of RAM. I've had this problem in the past and
eventually wound up reformating and reinstalling Windows. That, of course,
means reinstalling all my software. The problem just pops up all of a sudden
- no new software installations or add-ins. I first notice it in IE when I
type in a URL and press Return. At first it seems it's not responding but it
will eventually load after
about a minute and a half. If I click on an existing link in my history or a
favorite the site loads right away. At that point if I do anything that
requires Windows Explorer I have the same problem - it takes at least a
minute and a half to load Explorer to just browse, save or open anything. If
I click on either or try to close the windows while it seems to hang I get a
"Not Responding" message. When Windows first loads everything is fine. And
almost every time the problem starts after 30 minutes.It gets so frustrating
that I usually end up restarting. This computer is connected to a domain so
I've disconnected any mapped drives and disabled any login scripts with the
same result. If I do a diagnostic boot everything is fine. If I disable all
startup items and services except the Microsoft entries the problem persists.
If I disable all Microsoft entries except thsoe I can guess are vital for
Windows to load and include all other vendors it's fine. But to try to mix
and match the Microsoft entries would probably take the rest of my life to
get right. Also, if I login as a local user there is no problem. If I login
as a new network user the problem is still there. I've searched the web and
I've seen this exact problem or something similar but no solution that works.

Any feedback would be greatly appreciated!