The easiest solution would be to store the installation directory in the registry and look it up upon reinstalling.
To look up your registry value, you'd use something of the sort:
<Property Id="INSTALLDIR">
<RegistrySearch Id="InstallLocation" Root="HKCU"
Key="SOFTWARE\Company\Product" Name="Location" Type="raw" />
</Property>
If the registry value isn't found, the INSTALLDIR property will be set to your directory structure.
Rob has a complete solution on his blog for when you specify such a property from the command line.