In Device Manager, uninstall and reinstall the network card.
If that fails, make sure that the Network Connections Service is running. Check all the Services in "Adminstrative Tools" and verify that the services that ought to be running are running. Look for "automatic" services that should be running, but are not, and look at "disabled" services that should not be disabled.
Are you running in Selective Startup (MSCONFIG), or Normal ? Has this computer been infected/messed with ?
Read this thread:
The freeware "Dial-a-Fix" has a tool that will automatically re-register a bunch of your .dll's. I've had problems where doing this trick was this fix, so this is worth trying. If dial-a-fix doesn't do the specific .dll's mentioned in the link above (or any other source that recommends re-registering .dll's), you can do it manually. Read the thread in the link for some directions on how to do it manually (slow and tedious, but you will learn something...)
Also, dial-a-fix has a tool to show if there are "restrictive policies", which may not be your problem, but if you have them that would be evidence of past malware infection, something that would be good to know.