It's definatly coming from this site but not every time so amybe one of the ads.
Ths is the pop up box I got even thogh I have pop ups disabled and pop up stopper software:
The page at http://errclean.com says:
NOTICE: your system is not optomized and your computer performence is not at the highest level.
Full system optomization will greatly increase your computers performence and prevent data loss.
Woudl you lie ti install ErrClean to optomize your computer's performence now for free? (recommended)
OK and CANCEL both take you to the same site to install it