Doug,
This issue would appear likely to be a complex one, given that there are many cases where there is no significant slow down in either IE6, IE7 or Windows Explorer being experienced. For example, I have 3 machines on a network and access to another two on a different network. All are XP, but a mixture of XP Pro and XP Home, a mixture of SP2 and SP3. There is a mixture of CPU types, and there is no significant slow down on any of them.
The only commonality is that they all have 1GB memory, they all use Avast Home Edition AV, they all have Zone Alarm Free firewall 7.3.483.000, 3 have Spybot 1.6.0.30, two 1.6.0.31, and none of them use any 3rd Party add on Tool bars. Three have Teatimer 1.6.1.22 active, the other two don't use Teatimer. All have the SD browser helper active. Only one has any additional blocked items (only 4) added to the SD browser helper.
Your thought about having to start from scratch with a clean install of whatever version of Windows is being used, then add in Spybot and the SD browser helper and if the problem isn't present then gradually add in the other software may shed some more light. The problem will be finding someone who actually has the problem (or is it two different problems - one where the browser is affected, one where Windows Explorer is affected) who is prepared to do this in a very methodical way.