Since I am not there, this is difficult to troubleshoot.
1: Did the resolution change? High res on a not good enough screen will be blurry.
2: Monitors do get old and get out of focus.
I do not recommend this unless you are qualified, but most monitors have a focus control inside near the back.
If you remove the cover (powered off) and find the control, usually very small and uses a small screwdriver to adjust, you can boot (cover off) and adjust the focus back to sharp. I have seen some that had many controls for brightness, rotation, etc. If your test each one you can find the right one.
If you are not very careful, you will end up ELECROCUTED. This is why I say qualified. If it is a 17" and you are not prepared to take the risk, just buy a new monitor.
Okay, I have Windows XP and my problem just started yesterday. After I had went and turned on my computer, I got on the internet. Well, a couple minutes pass and I realize that my screen had gotten blurry (letters and everything). It happened today as well, and it's really getting on my nerves. If anyone can point me in the right direction and help me get my problem fixed, please by my guest.