Video-Editing Software

[almh]

Sorry for posting this query up here, but my current software is really bugging me. My dad bought me a Mac OS X, not entirely sure when he bought me it, but it originally came out about 7 years ago. The movie-editing software on it is iMovie HD, and was brilliant at the time. It currently is dying on me. The special effects aren't working, and even trying to delete clips from the timeline is pretty frustrating. Does anyone either know what on earth is going on with it, and how I can fix it, or any other good (non-expensive!!!) movie-editing software for mac?

I also should probably mention that the hard drive has died several times (and been replaced,) but that was due to the Sims, which I no longer use

If anyone does know how to get it fixed, or what replacement software I could get please let me know. I'm trying to get some new videos finished but it's an incredibly frustrating job. I am going to try the Mac help forum, but it seems pretty useless from what I've seen, and I only know one person that uses video editing software, but she's on a windows, so that ruled out asking my friends.