First make sure you have an anti-virus program on the machine. Make sure your signatures for the program are current using its update program. Always have the antivirus program running and checking for viruses. If it is a bought program, make sure you buy a new copy or renew the subscription when the subscription runs out. Usually programs have a one year subscription, unless it came with your machine and then it may be only 90 days. An antivirus program that is being used that the signatures are out of date is better than nothing but will allow newer viruses on the machine.
Second get an anti-spyware program. If it is one like SpySweeper or Microsoft’s program, that will run in the background and monitor for spyware, then treat it like your antivirus program. If you get one that just scans when you tell it to (Spybot, Adaware, etc), then run it at least one a month, or every couple of weeks if always on the Internet.
Third every few months run Scandisk to remove errors from your disk drive and run Disk Defragmenter to get all the fragments of files back in one file. This will speed your machine up. Both of these are found in My Computer on either your desktop or in Start, by right clicking the hard drive and choosing Tools.
Fourth your may want to get a registry cleaner (WinDoctor in Norton Utilities and other programs are also available) to clean your registry. This often clears up strange problems and can affect the PC’s speed also.
Lastly practice safe computing. If you get an e-mail from a place you do not know with an attachment, do NOT open the attachment. Don’t open attachments or links in any e-mails claiming to be from banks, PayPal, eBay, etc reporting security problems and that they need your password, account, Social Security Number, credit card number, etc. Opening the e-mail and not the attachment is normally no problem. Also do not ever supply this information on a web site unless you trust the web site. (Watch for a locked lock in bottom right of your browser to increase your confidence when you do know vendor) There is always the phone to use to call and place the order. If it is a real security issue the companies will contact you other ways than e-mail.
Enjoy your PC. I hope it is both fun and makes you more productive.