most likely they would have adware or spyware instead of a virus.
So if you run a program that cleans those out too you should be fine.
I like spybot - free too

I think if you download from the lesser known sites there would be more of a problem. I think yahoo should check those before offering them.
I know Google has some too- but some will say that they are from another source- as a warning.

all those things do use up memory etc- but as long as it doesn't slow things down you must be OK.

and some ad/spy/ant virus will miss what another will catch - so I run a freebie or trial one every so often to double check.