Are there any particular programs that people should avoid? Maybe the program is just lousy or it contains too much overhead and will slow down a computer...stuff like that.

Many people have become victim to scareware's which generates a false alarm in your system that you have viruses and asks you to download a specified software which in reality a technique commonly used to increase the sale of the anti virus. It would always be better that we avoid pop ups which advises on something about anti viruses.

I guess you have to be careful in downloading files, you should know if it is from a trusted source or not. Also be careful in using your pendrive and not sticking it to any computers for they may have a virus, Porn sites are also the must avoid sites.

Well, if you are buying a program (the physical product or from a reliable source), the first thing you have to take note of is the specifications. If your computer does not have the necessary hardware for an intensive resource hungry program (e.g. Photo or video editing software, or games), it is certain that your computer would be slowed down to almost a crawl.

If it is programs that are downloaded from the internet, you have to make sure that they are from a reliable source. Pop ups that tell you that your computer is infected, prompting you to download its recommended program is highly suspicious and best avoided. Likewise for downloading any file off the internet from a source reliability is in question.