Windows Aero causing lack of resources?

I hope the title is not to confusing. I did not think to take a screen shot or do a copy of the text that popped up.

What happened:

I got a new monitor this morning and so far its great. It works well with the applications I normally use.

Though I decided that I'd like to test to see how it is with games. I don't normally play Dirt 3, but I've got it on Steam so I thought I'll have a quick look and see how the GFX look on screen. I'll check some other games a bit later.

While the game was starting there was some tearing in the intro video which was a bit odd. I was a bit disappointed, though in the actual game play there was no issues. Only ran one track.
Once I exited the game there was a popup from windows telling me that my computer was running slow as there where not enough available resources and suggested turning off Aero. I clicked the ask again if this continues.

As it does not show my system specs:

i5 2500
8GB RAM
HD6870
LG IPS235 (I know the response time is not expected to be great.)

I know I've got a rather poor HDD, but did not think that it would cause any problems. I've played Skyrim on max settings in the past and Civ V with no issues at all. (different monitor though) as well as several other games.

Only things running at the time was Steam, Google Chrome and the game its self Dirt 3. Checked temps of everything with CPUID PC Wizard 2010 and everything looks alright, nothing sticks out as hot, infact they look rather low.

i get that message occasionally when playing bf3 and normally don't see a difference in frame rate if i keep aero on or off. i always assumed it happened on my end because of a bottleneck with my graphics card

Thanks for the fast reply.

I don't really need aero on at all, before I built this pc I always had Windows set to the lowest settings including plain black background and everything set to the Windows 7 Basic theme. Though I must admit it is a little nice to have it slightly fancier even though it has no use.

I hope nothing is bottlenecking already. It could be the HDD, though did not think that'll cause to many issues.

I've only ever seen this when running multiple screens and having the game on screen 1 and an aero-enabled window on screen 2.