First of all, let’s get rid of actual computer culprits known to slow down a computer. Some viruses (viri?) are known to slow a computer to a crawl, even as they wreak havoc on a system. If you suspect a virus, or other nasty unwanted beasty, it’s best to run complete virus scans such as Nod32 as recommended by KFI Techguy Leo Laporte, or avast! antivirus, as recommended by, ahem, ME!
(But Backup Your Computer First...)
It’s extremely important to back up all of your important data if you notice a sudden slowdown in your system, as this could be your only warning of impending data loss or worse, even damage to your hard drive (yes, some viruses can even cause real damage to the hard drive).
I recommend Acronis True Image Home 2009, because you can backup the entire hard drive, including the operating system. That's important; many backup systems only backup your data and not your operating system and programs! That means you have to reload your entire operating system, and all your programs and drivers and, oh my!--this can take days out of your schedule!
Acronis True Image Home 2009 backs up everything and you can restore an entire computer from scratch in 40 minutes! And, I don't get any kickbacks from them; I use this software! It's saved my behind many times!
Once you have backed up your computer, run your complete antivirus scan to catch or rule out a virus as the cause of a slow system.
How do you fix it? Well, that’s the harder part! Worst case, by uninstalling or rolling back the hardware or software (think, dumping the new friend!); best case, by downloading a patch to the software driver or application (smoothing over relations between the offending parties). You will have to do some digging and googling on this; I would suggest starting with the manufacturer of the hardware/software in question, checking their help system, knowledge base, and forums for possible solutions and workarounds.
Read on to Page 2 to discover the final, and most insidious, reason you will never have a fast computer (hint: it ain't because management is stingy! Wait, I take that back, that could be a factor!)