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Poorly working computers generally suffer from one or more issues:

A corrupt operating system. This is likely caused by loss of power at an inopportune time (less than 5% of the time), or the side effects of a virus infection (read, 95% of the time).
The consumption of resources to accommodate the not so helpful instructions of the virus corrupted computer.
The greatest common denominator of a poorly working computer is:

Illegal music sharing.
Visits to ringtone, wallpaper and “free stuff” websites.
Personally I don’t care how much money, Sony, Universal, Warner or EMI make or lose in a year. Do remember that not all sharing is illegal, however, it is VERY difficult to identify which sites and files contain a dangerous payload. In the “Wild West” of peer-to-peer (P2P) music sharing it is all less secure than i-Tunes, Amazon and other legal alternatives.

Likely to be safe music/video/software downloads

Not likely to be safe music/video/software downloads