JD123...I just purchased a DELL 5150 for £660 with kids and myself in mind.
Pentium 4 - 521 - 2.80GHz / Windows XP SP2
Memory 1024MB / 80GB Hard drive
CDRW/DVD-ROM / 17' FLat Monitor
Speakers with subwoofer (not really necessary)
Microsoft Office (Outlook, Word and Excel)
256MB Video card
1 year CAR
Am getting my own Antivirus package.
Obviously you can trim this way down for your needs and still get a decent machine although your 4-year will be using it before you know it (great educational software out there).
Not sure about video editing software. You could trim this machine down and use the money saved for that. Perhaps start a new thread about this.