How to decline the overload of hard disk

[Leon Wu]

Now I am making a program, which need play *.wmv, download, decompress
at the same time.
but I found when do those jobs at the same time, the overload of disk
can not be accepted, playing wmv is not fluent. further testing prove
when 2 of them run async there is no problem. because we have to
guarantee the effect of playing wmv. so I decide to sync the
downloading and decompressing. but those jobs are done by different
applications, they have their own job, I don't want to mess them. And
I don't want to modify too much. so I want to just sync the disk io. In
detail, I want to wrap the disk operation to make the io sync, and let
the downloading and decompressing use this class to operation disk. Is
this method proper, is there any good idea to solve this problem.
Any suggestion is great appericate.