Actually, Windows does tend to become sluggish over time, so instead of buying a new computer, just get ready to do a clean install.
1. Buy a new hard disk now. Not very expensive.

See what has a good GB/price; 500GB or 1TB.
2. Put the new HDD into your computer. If you need it, I'm here to talk you through that process. Then proceed to save all your important data in neatly labelled folders on that drive. Save data from your main C drive as well as the slave drive you think is about to go. Remember your documents, bookmarks, downloads, pictures, videos, yada yada yada.

Once you format and reinstall, they are gone, so be thorough.
3. Think about installing Windows7 64bit. I rather like it.

Go to the website of your computer manufacturer and download all the latest drivers for your model, and put them in a folder ready to use. Burn this folder to a CD/DVD for later. Just to be thorough, do this for any other version of Windows you are considering installing.
4. If you are using any legal software

remember to backup your licenses for each of them. Ask them now for help if necessary. They will not help you once you have formatted the HDD unless you have proof of purchase, etc., so get help now so that you will be able to install them again.