If you have a separate drive/partition, change the store directory to a folder on the other drive/partition. Then backing up won't be an issue, all you'll have to do is change the directory to where you saved everything after each Windows install and it will automatically put everything back. I'm not at home atm, but if I recall correctly, it is Tools->Options->Maintenance->Store Directory.
You can also use the export option under File->Export (you can also back up your address book) and then obiously you would use import to bring them back after install.