I switched my parents to Ubuntu a while ago, but they still need a windows installation to run their accounting software on. What we did was install the accounting software on a windows server, then they accessed it through remote desktop from their ubuntu box and ran it remote. Has worked like a charm.
All we did was to enable remote desktop for their users on the windows box, installed tsclient (
Terminal Server Client [tsclient] | Get Terminal Server Client [tsclient] at SourceForge.net) on the Ubuntu box and having them connect through tsclient to the windows box with their windows accounts.
Not the fanciest solution perhaps, but it works. Maybe you want to give it a try.