Hello, IamR.Ph
thanks for the info.
first "clean installation".
Look here
http://support.apple.com/kb/HT1545, and scan for "Erase and Install". You must have the installation disks, i hope, also for the system; they go with each macintosh. This "Erase and Install" means that the hard disk will be newly formatted, and all previous info will be wiped out. I would do that in any case if i bought a computer second hand.
There is also the possibility to do an "Erase free space" in Disk Utility (in Applications->Utilities). This erases not only the file names from the directory, but writes (one or more times) "0" and "1" over the entire "free space", and really eliminates the previous information there.
However, your 2nd mail contains info which makes it very likely that it is not the free disk space on the hard disk, but something else. You seem to have 4 GB RAM, that's good, and most of it is free... The changing numbers depend on the status of the different processes. Behind the screens a lot of tasks are going on which are hidden to the users.
all this sounds ok to me, so there is another problem. I have no experience with iMovie, so cannot really help on this point. You might want to wish to repost in the iMovie section, which is here:
http://discussions.apple.com/category.jspa?categoryID=141
Still, i would make sure that nothing of the prvious owner is on the disk.
Success, and sorry that i cant help
Thomas