Thanks for your post.
Have you tried IE 6.0 or IE 7.0 or other web browser from the SBS 2008 server?
Does this issue occur on other computers when trying to access the companyweb and then Excel file?
If the issue only occur to IE 8.0 on SBS 2008 server, please try to reset IE 8.0 first:
Reset Internet Explorer (IE)
1. Click Start, please type "inetcpl.cpl" (without quotation marks) in the Start Search bar and press Enter to open the Internet options window.
2. Switch to the Advanced tab.
3. Click the Reset Internet Explorer Settings button.
4. Click Reset to confirm the operation.
5. Click Close when the resetting process finished.
6. Uncheck Enable third-party browser extensions option in the Settings box.
7. Click Apply, click OK.
Here are some articles from SBS blog which describles similar issue for SBS 2008 instead:
More related information for SBS 2008
Files in Companyweb are Opening Read-Only After SBS 2008 UR2 http://blogs.technet.com/sbs/archive...-2008-ur2.aspx
Known Issues After Installing IE 8 on Small Business Server 2008 and the Vista Clients That Are Joined to the SBS Domain http://blogs.technet.com/sbs/archive...bs-domain.aspx
Hope this helps.
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