If someone calls up purporting to be from Telstra or Microsoft (or anyone really) and offering to fix your computer please don't let them remotely access your PC and "fix" it for you. I know that sounds obvious but they play the numbers game in saying that they are aware that you are having issues with your PC or internet connection. Inevitably some of the people they will call at any given time will, in fact, be having issues just based on probability.
Having just fixed one of these issues in the last 24 hours I can assure you it's a significant and unnecessary expense to reverse their "fix", get your data back and restore your computer to a working state.
If anyone calls you offering to fix your PC I suggest you just politely decline and hang up on them, also be prepared that they will probably persist.
Also seriously consider NOT being an administrator on your own PC and instead have a separate administrator account. Also, never store banking, credit card details on your PC just in an unencrypted document.