It's the HD video that might overtax the 2.2 Ghz CPU, but that depends on the acceleration available in your GPU solution...however, if she's NOT complained about video viewing as the speed issue, then I suspect she's experiencing configuration / disk oriented problems.
Not errors, just fragmentation / configuration performance issues.
Check, though, by using task manager or similar software to see if the CPU is taxes to 100% on any single thread (or the whole CPU) at the point(s) where it's performance is in question.
If the CPU is below 50% when it's being slow, that's disk.
I've had many situations where a switch from X2 to Quad with a 1Ghz jump in speed produced no measurable performance improvement because the same OS and disks were re-used.
That drive isn't bad, though - it's just not what the newer offerings in the black line would do.
If the CPU is taxed, and you upgrade, you might STILL need an OS cleanup and disk upgrade to get it working faster all around.
Try a quick experiment if you can....find a spare drive and install a new OS on it...just to see if the machine "wakes up" doing that...minimal application / os configuration...do the web browsing and an HD video playback to see.
You might find a direction that way.