I get a message "start up disk is full" Does this mean I have too many programs running?

No, it means exactly what it says. There IS NOT enough room on your startup disk to perform whatever task you're attempting.

What's running ISN'T the problem. It's what's physically stored on your hard drive.
I learned several yearas ago NEVER TO LET a hard drive get more than 75% full at any time ever for more than a few minutes at most. You're BEGGING for trouble if you do.

Regaining Hard drive space:

The easiest approach is to assume that the space has been filled up by your own files: documents, but also includes, videos you have downloaded, photos, music, pdf's, downloaded software. As these files have either been created or downloaded by you, you will know where to find them and these are the things to move or delete.

So look in your Movies folder, Documents folder, in the iTunes and iPhoto applications, for excess movies, photos and music that you can move onto an external drive, or burn to a CD or DVD, and then delete.

After you delete anything, remember to empty the trash to recover the space on your hard drive.

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