Spyware is the most common type of infection, but also the least worrisome. Spyware encompasses everything from a cookie that reports too much information, to a add-on to your browser which will redirect your Google searches. Spyware tends to be extremely subtle and most computers will have some form of spyware on them even with the most up-to-date anti-virus software. This is because the line between legitimate data mining and illegitimate data mining is rather thin.  You can keep yourself clean of spyware by running a freeware program such as Malwarebytes.  This program will not run automatically but will have to be rung on it own to scan the system.