Regarding passwords, use random passwords with letters and digits. I have a spreadsheet with all my passwords. They are all random and I never re-use them. Many people not only use simple passwords, but also re-use them. So if one account is compromised, they all are.
For example, some small poker sites have disappeared and taken all the money. I have had that happen to me 4 times already. Had I been using the same password, I would have been very worried. If they are capable of stealing my money from their site, they are capable of stealing it from other sites as well.
Regarding concerns to get viruses and malware, and so on, if you really want to be sure, don't use your poker computer for Internet or email(except for web-based email). You would either need a 2nd computer for Internet, or use virtualization software like Vmware. The latter is a program that allows you to create virtual computers inside your physical computer. If one of your virtual computers gets infected, you simply delete it and copy your fresh version in its place.