I'm only a computer guru if you are challenged with computers, but may be able to help.
I have the Norton Internet Security package. It is good, but has a lot of bells and whilstles I don't use. You can be safe without it.
As others have said, get and install XP service pack 2. Can do as a long download or order a CD-ROM.
Add a second spy ware program as each one catches different things. I use Spybot and Ad-Aware second edition and they work well in tandem. Both are free.
Train your wife to turn off cookie enabling if she is surfing non-trusted sites. (Usually found under "tools", "Internet options" on your browser.) If you need to allow a cookie to get to a site you want, allow that cookie then clean up afterwards. Ad-Aware will clean out cookies.
Switch over from Internet Explorer to Firefox as your browser. It is a free and easy download and install and, thus far, much less vulnerable.
Zone Alarm and Black Ice Defender are both free firewalls. I did have a problem with incompatability with Black Ice Defender on my last computer, but Zone Alarm caused no problems.
I use all of these, and more, and have yet to get a virus. Of course, the best anit-virus tool is your brain. Be very careful on what attachments and e-mails you open!
Good luck!