[Dan Zollman]

When you use a part from the Content Center, it automatically saves that part--unless you change the defaults--in a special Content Center folder. This way, when you open the assembly, Inventor can open and display that part file quickly instead of going through the Content Center/Design Accelerator every time. This means that if you move your project folder to a different computer, Inventor will not be able to find those parts (because it doesn't have that Content Center folder) and it'll give you those errors. What you need to do is find that Content Center folder and move those files to some place within your project folder.

This folder is usually located in the user Inventor directory where you save your project files. On my computer (default installation setup for R11), the Content Center folder is:
C:\Documents and Settings\Daniel Zollman\My Documents\Inventor\Content Center Files\R11
Move those files to your project folder. You will then have to redefine the file references within your assembly, either with Design Assistant, or by going into the assembly file and Replacing each bolt manually.

There's one more way to store Content Center files inside your project folder. Assuming that you're working with a project (.ipj file), you can:
1. Go to the Projects window (File>Projects)
2. Select your project on the list
3. Click the plus sign next to Folder Options
4. Specify a new path for "Content Center Files" to something inside your project folder
That way, the next time you add a part using the Content Center or Design Accelerator, it will automatically save the part in that folder. When you move the project folder, the parts will move with it.

Sorry if my instructions are incorrect for Inventor 2008--I'm still using R11, but I can't imagine that Inventor 2008 is significantly different.