Don't buy a USB adapter unless you absolutely have to. They are such a pain that they are a last resort only.
For Desktops, buy a PCI card.
For old laptops that are too long in the tooth to be wifi-enabled already, buy a PCMCIA card. Note that these are also known as PC cards. Some people refer to them as PCMIA (without the C) but this is incorrect and could be misleading.