To answer your question, Salima, I did; when AVG added its anti-spyware module (which they purchased by buying out an independent company, ran for some months as an independent application, and then tacked on the their own AV modules), I noticed---at least on my computer---some sluggishness compared to the prior release. But then it was trying to do two jobs instead of one, and the "marriage" of the two modules doesn't appear as robust as it might be.
There are three things you need to consider in choosing an AV:(1) detection rate, (2) frequency (and availability) of updated definitions, and (3) ease of use. The reports I have seen put Avira above AVG in both (1) and (2); you must decide for yourself which is easier to use, and makes the best sense for your computer and how it is configured.
The best defence is a layer of security applications, beginning with a software firewall, then a good AV, and then one or two anti-spyware applications. Using a browser other than Internet Explorer (which is subject to continual new threats because of its widespread use), is also added protection.