One of the most overlooked and underemployed tools for the home computer user is an antivirus/preventive maintenance suite and specific software programs to kill unwanted uninvited visitors.
I recommend System Mechanic Professional
which in almost 20 years of PC repair/upgrades/custom building I have found to be about the best program available in this area. Outperforms Norton 100 to 1, McAfee 200 to 1 and everything else.. well just not worth the initial cost to purchase. I have been using Iolo for almost 5 years now and will not change. Install, configure (the how to's are really easy) and SMPro will keep an eye on your puter. No, it will not tell you that you have a bad memory chip but -- if you know how much ram you actually have, when SMPro recovers unallocated memory that Windows will not return once used, you will see how much total good ram you have available. If there is a difference simply subtract existing from known max and you can pretty well determine if you have a bad chip. Also Iolo will update regularly to keep your definitions up to date when properly configured. Iolo is one of the few who take care of existing customers and offer a reduced rate .. 20% to renew at least 31 days in advance of expiry or 10% prior to expiry. That is good in my book.
I also recommend Spy-Bot
as a must have add-on. Install, run and it will detect any malware on your computer. It is about 95% effective. I use a couple others as well and will post links to them in a bit. Used on a weekly basis, Spy-Bot is really useful. Don't forget to Immunize and to check for updates on a regular -- weekly is good, bi-weekly is acceptable to keep your definitions current.