"The ordinal 421 could not be located ..&quot

Hello,

Okayy, my computer is seriously SCREWED in some way that I do not know. Every time I restart my comp, I get this message saying I have limited space. But then, there's another error ..

" The ordinal 421 could not be located in the dynamic link library urlmon.dll. "

Apparently, it started happening after I uninstalled IE 8 because I accidently put Opera as my default browser. But I get this message that pops up saying "Opera.exe corrupt file." and wouldn't open Opera.
I know, I was an idiot to accidently put it in default, -face palms- but I really need help. I'm not a genius at computer networks, fixing viruses and redoing the things that cause major problems to my PC. But at least I could try. My older sister tried helping me by trying CCleaner because I read off some forums that it could work.

I already went to Safe Mode but I still get the ordinal 421 error. And when I try to System Restore, I couldn't continue to finish the restoration because I COULDN'T CLICK NEXT ! I don't know what to do next but either get rid of my computer or bring it to some expensive workshop to get it fixed, which I don't count as an option.

I know I'm blabbering but here's some info on what happens when I restart my PC:

-Logs on Computer Account, NO Start tab or icons/graphics. Only my background.

-Whenever I log on any account it gives me this "limited error" window which I click to close. But appears every time I get on my computer.

-Using System Restore does not work and so does Safe Mode. Currently, it's pointless.

-My computer can sometimes suddenly reboot itself when I am at the point of getting onto my Computer account and can repeat for some time.

I know that this could be a severe computer issue, but I'm not REALLY intending to spend a fortune on fixing my computer .. I don't even know if my computer if beyond help, so what can I do to fix it?