I've got an HP DC7100 and an HP m8530f. The DC7100 is a small form factor desktop while the m8530f has a mATX board with lots of extra features like front I/O and HP Personal Media Drive bay. Both of these have very little space (especially the DC7100) and don't have any other places to mount fans.
What other possible ways of cooling are there, if there isn't much space left inside the case?
Thanks in advance.
EDIT 1: Has anyone tried an expansion slot exhaust fan before?
I have had to migrate systems out of their cases and install them in larger cases due to this situation. We have 20 Dell ultra small form factor systems which were getting so hot, you would literally burn yourself on the hard drives, which sat directly above a heatsync on the motherboard (not the CPU, maybe southbridge or chipset or something) that gets extremely hot and traps the head below the hard drive. About 6 months after these computers were out of warranty, the hard drives all started going bad. We took the guts of each computer and installed them into standard ATX cases with much larger cooling fans. We were even able to upgrade the memory on them without worrying about overheating.
My suggestion is to migrate to a larger case.
If you're not above case destruction (read: past end of warranty), cut more space for air to flow in your existing vents.
I had a friend who would remove the fan grilles on the back of his case. Since he was running under-voltaged fans (or low-power "quiet" fans) he wasn't afraid of touching them while they were moving. (I have extremely inquisitve cats, so I remove the case grilles and replace with a wire grille.)
You may also want to see about plugging up unnecessary vents as well. This sounds odd but if you can get a clear single path for air to flow through the case you can generally get very effective cooling.
Beyond that, you're starting to look at dryer ventilation hose and other such radical measures to direct air. My friends and I would use that to make the PSU fan pull double duty as the CPU fan (only in lower-heat machines like a Pentium II; I wouldn't do this on a heat monster like the Pentium 4 or Power PC G5.)
We usually optimized for noise not cooling, but you can usually improve both over manufacturer spec.
...or you can just submerge the entire thing in high-grade vegetable oil and run your cooling like that.
Try a simple approach that is just to have a small regular fan near the PC to pull the warm air away from it.
I myself make use of a large 30cm floor fan to assist in cooling my desktop machine on warm days when it heats up and find putting the fan near the back of the tower helps cool air circulate better into the case.
