I am working on a Dell 4550 and was having all kinds of problems which I thought were causes by a virus.  Then I opened the case and found lost of accumulted dust.  I cleaned it out with a can of air.  Now the computer will not boot up at all.  The lights indicate a memory problem.  A is green, b,c, and d are yellow.  The beep code are 1 long beep followed by 3 beeps and a pause then 2 more beeps.  I purchased a new memory module and put it in, but have had the same results.  It doesn't matter which of the two slots that I put the memory in.  I used a known good chip from my computer with the same results.