Hi,

your grokster obviously installs some adware or spyware on your PC:

either you don’t use grokster any more (alternative: kazaa Lite)
or
you configure avast mainscanner & resident Shield to exclude the relevant Grokster/adware files from scanning. Search through the options/program help for this

please give us more info, which virus/trojan was found, and where (full path/folder/filename); see avast’s report/log