Pushdo (Cutwail) infected millions of computers during 2009. a New version is active currently and this version also hides botnet control commands in HTTP requests, similar to previous versions but the difference now is that it targets hundreds of smaller legit websites and no longer sites like Google, CIA or the FBI.
If your computer was infected with this malware you would only notice a slightly slower computer. The best way to avoid infection is to constantly patch everything and definately Java and Flash, Pushdo installs rootkits and other software which is hard to detect with anti-virus and anti spyware software.