Removing the side panel and placing a desk fan blowing in can work temporarily.
It'd be cheaper and easier to replace the fan instead of the entire computer, however.
No, it can't.
A closed air case is designed with three things in mind. Air flow, Dust Prevention, and EM Blocking. By removing the side of the case, you're opening up a hole in the case, which screws up the internal air flow, allows dust to get in at an accelerated rate, and you now have a huge section where EM shit can get in. Fans of any kind generate an EM field with their spinning, especially if the blades of the fan are metal. All three of these issues are Bad for Business when it comes to PCs. Disrupted air flow causes things to overheat faster, because the air is not flowing where it was supposed to, Dust gets in quicker, builds up quicker, and causes heat to build up on parts quicker. And well, any form of EM is bad for any electronics. And if you demand references for this, you won't get them till after May when I move and have found my A+ Ceritifications text book, and my text book from Intro to PC Construction and Management textbook.
As for the PC, it does indeed need an upgrade, which will be coming about Mid May after I get moved, which will be a whole new computer, new case, new AMD Phenom II or maybe Athlon II quad core CPU, and a couple of other nice goodies, really depends on what kind of sales TigerDirect has after we get moved come May 1st. I'll find a way to keep this one running, or I'll just switch back to using the laptop as my main computer and this one as the browser PC that does nothing but have Firefox open at any given time. Although it's going to be rather difficult to switch back to the 1024x765 resolution of the laptop after having the 1680x1050 of the PC for so long. =/ But C'est la vie, and Que sera sera and what not.