Before even writing on how this can be done I have to state that I would not recommend this be done on a production system. Previous versions of any software is going to leave traces of itself even after an uninstall so, while everything may seem to be working fine at the time, issues may occur down the line from this type of workaround.
While trying to install SharePoint RTM version, the setup program may produce the following error if a previous version was installed; "The install in progress conflicts with a previously installed Microsoft Office 2010 Server product" Fig. 1. To find out how this error can be fixed, first let’s look at how a Microsoft Installer setup checks for previous versions of a program.
When the setup for SharePoint 2010 runs the Windows Installer components it checks for previous versions. These previous versions will usually be listed in the Control Panel->Programs and Features in Windows 2008. If an update has been installed for a product though, it may be listed in the “View installed updates” link in Programs and Features. Fig. 2.
When the View installed updates is clicked the Programs and Features will change to list only updates to the Programs installed. Updates listed here can be uninstalled. If any SharePoint updates are listed click them and uninstall. Fig. 3.
After uninstalling all of the SharePoint related programs from both “Programs and Features” and “View installed updates” reboot the computer and try the install again. If the setup is still finding the previous versions check that there are no programs listed in the registry under the following key: HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall. Look first for any key with names similar to the ones in Fig. 4. See the following KB article on using the Windows Registry Editor for the warnings on usage; http://support.microsoft.com/kb/256986.
For each GUID found ending in 0000000FF1CE in the HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall, find the UninstallString value and copy out the data by double-clicking or right-clicking and selecting Modify. In the “Edit String” window copy the “Value data”. Fig. 5. This is the command that will uninstall the package. Next, open a command prompt as Administrator by going to Start->All Programs->Accessories->Command Prompt; Right-Click Command Prompt and select “Run as administrator”. You may have to do this multiple times to remove all of the Office related registry entries in the uninstall key.
After all of the entries are removed remember to reboot the computer, as the MSI will more than likely need the reboot to clean up all of the in use resources and may not let you spin up another install until a reboot is completed.