Programs won't open until others close

I'm experiencing a really frustrating bug (Vista Home Premium) that will only

happen occasionally, but when it does, it persists until I restart my

machine. Basically what happens is it gets in this mode where it will not

allow me to open another program, no matter how many times I try to open it.

However, if I close an open program, whichever program that was trying to

open will then open.

Why is it doing this, and how can I fix it??

Thanks