It does depend on how rotten your sills are, but there are liquid epoxy resins you can apply to fill gaps and voids, or epoxy pastes to repair patches. There are useful step-by-step instructions in Save Your Wood Windows and Wood-Epoxy Repairs, priced $15 each, both by John Leeke and available from his website (www.historichomeworks.com). Or try www.timber-repair.co.uk or www.paintz.co.uk for advice on types of repairs and product. As for your bugs, go down to your local hire shop and hire a hand pump spray. The hire shop will also sell water-based dry rot and woodworm treatments that can be sprayed on to the wood.