Firefox Downloaded By A Cold Caller

Last week I received a call from someone allegedly working on behalf of Microsoft who had reported that my computer was generating many "errors and warnings" that I couldn't see.He told me that for £37.45 he would download Firefox which he stated would be the safest way to access the internet, I agreed to this but feel a little uneasy having just seen a news report about cold callers downloading software which they can then use to access my personal details. Am I now at risk or was this a genuine call?