You should consider keeping your important (e.g. bitcoin/litecoin/namecoin/etc) wallets on one virtual machine, and running new/shady alts in another.  I'm actually surprised more people don't do this, as it would prevent 99% of the theft incidents I've seen.

Never use either of those virtual machines to browse the web or do anything else outside of working with those wallets.  Never install any software that you don't absolutely need on them.