You dont have to but it is a good idea if you want your computer to run a little faster & most importantly so your friends dont see the porn websites you have been on J/P but yea its a good idea to do so
Antivirus 2009 is a VIRUS itself it is made to look like a real antivirus program but its function is the complete opposite
normal antiviruses probably wont work to remove it
u need to download hijackthis and malwarebytes (google or pm me for links)
you should run a malwarebytes scan to be sure you have removed everything.
pm me if u need more help