Well, at 3 years old, I'd replace it unless it's hopped up for gaming. The HD has 3 years of the mechanial running and as others noted it may have sectors starting to go bad. Any 1/2 way decent MOBO is $125 or better with the rest of the componets at 3 years use. If you have a bad stick of RAM cheap enough to replace if you can prove it's bad. If the OS load has never been reloaded it's way over due.
Even a basic Dell is only $450 (many were cheaper than this one) for the basic office/school and internet surfing. And oh by the way at least 1 year warranty. I would think hard about dumping $$$ in a 3 year old box without knowing the exact problem. Reuse your monitor to save $$$.
PS - Not pushing Dell, just the easist to pull up numbers for.
Inspiron 580s
Powerful Desktop with a Stylish Look in a Slim Design
Intel® Pentium® G6950 processor(3MB Cache, 2.80GHz)
Genuine Windows® 7 Home Premium, 64bit, English
320GB
2 - 7200RPM, SATA 3.0Gb/s, 16MB Cache
No Monitor
Microsoft® Works 9 (Does not include Microsoft Word)
Intel® Pentium® G6950 processor(3MB Cache, 2.80GHz)
Genuine Windows® 7 Home Premium, 64bit, English
320GB
2 - 7200RPM, SATA 3.0Gb/s, 16MB Cache
No Monitor
Microsoft® Works 9 (Does not include Microsoft Word)