Could one of these devices be overheating? Blow the dust out of all the vents, get them off the carpet if that is where they sit, and ensure they are getting some decent airflow.
So both of those boxes multi-boot all of the listed OS's? Are you using (ie, do you prefer) simple sharing or advanced sharing? Are you comfortable with the idea of disabling all security within your LAN?
@Ponch: that particular video is blocked in the USA but I was able to download other videos from that site with JDownloader. Just copy the URL and it parses out the download fine.
I see a previous suggestion of using alternate file managers but I'll also add that xplorer2 has this behavior as a toggle-able feature. http://www.zabkat.com/
Your DNS situation is interesting but shouldn't impact the ability for two machines on the same subnet to be able to ping each other. Will they both reply to pings from the host machine (not VM)?
Here is a tool that lets you view this in effect and make changes: Known Folders Browser 1.0 (for Vista and Beyond) http://weblogs.asp.net/kennykerr/archive/2006/11/02/Known-Folders-Browser-1.0-_2800_for-Vista-and-Beyond_2900_.aspx
Perhaps this will suit you? http://lifehacker.com/5898632/netspeedmonitor-keeps-your-network-activity-in-the-taskbar-shows-you-bandwidth-reports-on-demand
Please show us the output of "ipconfig /all" as you may have a bad gateway address but that is rather rare. Have you tried changing from DHCP to static? Is there an IP address collision?