I only have win8.1 to test my dreams and all that I post have low cpu use.
I run a CPU gauge fulltime on the desktop to check all dreams I make before upload to the site.
also the moderator checks them.
Win7 has been awhile back so not sure for you what the problem is.
WMV dreams do run a little higher CPU use than AVI but not that much.
After 100's of Dream uploads I get few CPU problems back, might be something with your PC.
I do note on my uploads which file type I used so maybe you should just stay with AVI dreams ?
Those 3 WMV dreams run at about 20% cpu use and the AVI run about 10% or less on my win8.1 PC
Try checking windows explorer and see if it's running to high or multiples and click on restart then check your CPU usage.
For whatever reason windows explorer goes nuts and runs high for other reasons other than running dreams .
Az