I Have been blocking a lot of emails directing me to porn sites, what happens to these blocked emails? do they store up on my PC or what? and if so will they eventually slow down my PC? I saw a question on this about 6 weeks ago but have been unable to locate it.
You haven't said what method or software you use to block them. Depending on what blocker you are using and how it is configured, they may:
a) Never reach your machine
b) reach your machine and be deleted
c) reach your machine and be stored in a a "junk" folder.
Contrary to the beliefs of many, storing stuff on your machine does not slow it down.
Storing stuff in your mail folders may slow the opening and closing of your mail client (and searching for old messages), but that's all.