Unfortunately, a Pentium 4 is probably not nearly enough real world power to run Premiere Elements 8. But that depends on
your source footage. You may be able to work with tape-based video -- but you're certainly not going to be able to edit AVCHD or hard-drive camcorder-based video, which usually requires at least a quad-core processor.
Meantime, do go to Premiere Elements 8's Help file and select Check for Updates to ensure you have the version 8 patch. That may help a little.
But you're likely way underpowered -- especially if you're trying to edit anything but tape-based video. Sorry.