Try this.
Although my sample effort is not that spectacular is was knocked together in about 10 minutes. with a bit of time and effort you should be able to tweak it so that it is much better.
The sample I have used is of my wife bobbing about in a helicopter, I wouldn't think the number plate you are trying to disguise will be bobbing about as severely as the sample I have used.
Firstly download
this small zip file and extract it so that both files are located at
c:\Projects\blur01.vsp
c:\projects\blur02.vsp
Edit:
VSP file for VideoStudio 9 users here.
Now download the file 03.wmv from this link
http://www.mediamax.com/sjj1805/Links/21ED3FFB34
Please do not at this time download the other files.
My MediaMax account allows me to share 1GB of stuff per month and every time someone downloads something it eats up a bit of that 1GB.
Place it in the same directory as the two vsp files above
c:\projects\03.wmv
Now open VideoStudio and then open blur02.vsp
Briefly what I did was to take the clip of my wife and place two filters onto the clip to act as a mask. This was then saved as blur01.vsp.
I then started a new project and placed the same clip onto the timeline with the modified clip on the overlay track.
