A trojan is a serious security risk. If you have used your PC to buy anything on the internet and supplied credit card details online, you might feel it worthwhile to ask for a new card. If you have registrations on various places, for example email addresses, you should change all your passwords. You should never, ever use the same password on different sites, although using the same user id is less of a problem. For example, you could contruct a password for each site by using (say) the first three letters of the site, followed by a significant number backwards (your birth-year) for instance), then a fixed password (1st two letters in upper case).
Here is an example for this site "tso6591EXhonour" Now you see how easy it is to create a memorable, yet hard to crack password which is unique to a site.