Are you using Internet Explorer?
You may have your IE zoomed-in (which can anti-alias images, thus making them appear a little bit fuzzy).
Have a look in the bottom right-hand corner - you should see a magnifying glass and a percentage figure - 100%?
If it's more/less, try setting it back to 100%.
Of course, it goes without saying that you should ignore the above if it is not applicable!
Cheers,
Shaun