Everyone has an old computer somewhere. Just reinstall that one cleanly, install AV software on it and only use it for bitcoins. If you do not use that computer for surfing or other things, it is perfectly secure for storing your wallet. Have bitcoin-qt on there and - if you have a lot of bitcoins - store them in Armory on that PC. Use that for your savings.
You can have a bitcoin-qt on your everyday-use computer for your pettycash. If you are a miner, let the pool send the credits to your pettycash account. When there is some value in there, send it over to you dedicated savings BTCPC.
If you need some bitcoins for spending, just send it from your savingsBTCPC to your everydayPC's pettycash wallet and do your payments with that one.
With Armory, you can even PRINT ON PAPER your private keys. No more backups, you have your private key printed out! (or save it on a dedicated USB stick you do not use for other things).
What if all your systems die on you? Get a new computer, install bitcoin-qt. Let it update the bitcoinchain (yes, wait a couple of days, unfortunally). Install Armory and fill in your private keys from the printed paper. Your back in business.
If you make sure your bitcoin-qt wallet on the savingsBTCPC is empty (ie, all is in the Armory) and you have little or none BTC in your pettycash PC (or have a backup of the wallet.dat), you do not lose a dime.
Onlinewallets, suite yourself. I do not want to be depending on third parties security settings. I want to have it in my own control. Ok, I have some BTC and Euro's at MTGOX. I know.... but that's just my pettycash.