Even a system pretty that is pretty badly compromised is probably still going to be safe for you to use if you are using a hosted (shared) EWallet service where a one-time password (OTP) like Google Authenticator (on your mobile) is required.

But overall, it is really a matter of giving attention to security relative to the risk.

If you carry $200 in your back pocket in a bar, you probably will be fine with $200 worth of bitcoins on your Bitcoin.org client on a Windows Vista or Win 7 box presuming you run updates, have some decent anti-virus installed, are sane when installing software, and know how to copy your wallet.dat to a thumb drive for backup.

Now if $200 worth is a lot for you, or you want to store $1K or more you might not want to trust that configuration.  An offline system (with an air gap to the network) booted from a LiveOS distribution is among the most secure methods for using bitcoin -- but that security comes with the tradeoff in convenience.