Most people will tell you Spybot and Adaware are outdated products that are not up to the task nowadays. Superantispyware
and Malwarebytes Antimalware
are two excellent free on-demand antispyware scanners.
According to AV-Comparatives McAfee with Artemis has oustanding detection, however, it is not a good choice for old computers as it uses a lot of system resources. AntiVir (free or paid), Nod32 and Norton AV are some AVs which provide great protection and is very light on system resources.