You might think about replacing the computer if it's more than 3 years old. This seems drastic, but I could kick myself for not doing that a year ago. I got a new laptop for business use and it is incredible. 10 seconds from power-on to the Windows interface. I use Norton 360 on my machines. It's a little expensive but can be installed on multiple PCs (I think it allows up to 5). The best thing is my sister-in-law got one of mine infected. I called Norton and they spent about 1 hour cleaning it up at no charge.
I also got a Chromebook last year for travel use. That is a slick machine. It doesn't do everything exactly like Windows does, but they are very inexpensive (<$200), very light (2 lbs) and the battery lasts 10 hours. The Google office apps are free, so if you don't need to interface with Microsoft Office from other people, if you mostly browse and write a few letters and some spreadsheets, the Chromebook is an excellent option. My 7 year old grand-niece is getting a Chromebook from the local school for 2nd grade - I guess it's cheaper than buying books and functions fine for their needs.