If it was a virus consider to use linux instead of windows. It is very unlikely to get a virus with linux.

Never give your private keys to anyone and if you use a web wallet, bookmark the legit website and check the URL and SSL certificate before you login to avoid phishing.

It is better keep your keys offline (in password encrypted text file with secure backups) and sign all transactions offline. Here is a guide from myetherwallet about how to make offline transactions: https://kb.myetherwallet.com/offline/making-offline-transaction-on-myetherwallet.html