I'd suggest that you use your browser's controls to zoom in on the page until it's perfectly readable to you. On most Mac browsers this is done through ⌘+, and I guess on Windows machines it's usually alt+ (or maybe it's ctrl+. (If you want to go the long way round, the command to "zoom in" is almost always under your browser's "view" menu.)
Then, keep reloading the page until you get a captcha that you can definitely read. Eventually one will pop up that will be pretty clear.
I'd also suggest that when you're creating your account, make the password stupidly easy — some word or number sequence you definitely won't screw up. Then, after your account is successfully created, you can then go back to Special:Preferences and change the password to something more secure.
Give those suggestions a try. Whether they work for you or not, please come back here and tell us how you got on.