Use G-Spot to determine the codec. What camera is it?
Start with these links on the RIGHT SIDE of the Premiere Forum
How to ask a question http://forums.adobe.com/thread/416679
Premiere FAQS http://forums.adobe.com/thread/424418
Premiere Tutorials http://forums.adobe.com/thread/424009
PPro Wiki http://premierepro.wikia.com/wiki/Adobe_Premiere_Pro_Wiki
PPro Help http://help.adobe.com/en_US/PremierePro/4.0/index.html
Also, A link with many ideas about computer setup http://forums.adobe.com/thread/436215?tstart=0
Work through all of the steps (ideas) listed at http://ppro.wikia.com/wiki/Troubleshooting
If your problem isn't fixed after you follow all of the steps, report back with the DETAILS asked for in the FINALLY section, the questions at the end of the troubleshooting link... most especially the codec used... see Question 1
Welcome to the forum.
There are dozens of similar threads, but many have different causes. Usually the key to deciding which is the cause for one user vs another, is the camera in use.
Often, it will be something like the CODEC being used by the camera causing the issue. As an example, footage from helmet-cams, Web-cams, some miniature video-cams and many multi-format DSLR's, will use CODEC's, like the Xvid to heavily compress the footage into tiny files. These are designed to be viewed and uploaded directly to YouTube, and not to be edited.
That is why Harm has asked for the make and model of the camera. It would be very helpful if you would also look in the manual to find exactly what the specs of the footage are, as mfgr. Web sites can be really difficult to find those specs. Harm also mentioned G-SPOT, a great, free utility for gathering data on AV files. That is one of the most useful tools that one can have.
If you can tell us everything about your footage, we can get to work finding out why it is Importing, as it is, and can make recommendations, or point you to specific threads with cures and fixes.