How old is your PC? If it's a few years old there might not be a point to seriously upgrading it, it might just be cheaper to get a new one (with the low prices of PCs these days). I full reinstallation of Windows would be ideal though, and that's what the repair guy probably did last time. You'll need your original Windows installation disks for it though, or a restore disk if it came with your PC. This will wipe everything from the computer though, and start again fresh.
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Intelligence is the ability to avoid doing work, yet getting the work done.