CPU and RAM intensive software or security suites that you should avoid are generally the big names. McAfee, Sophos and Symantec products are designed for successful use on high-end PCs rather than the self-build your neighbor gave you. As such, you should be looking at one of the following options:

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Avast – available completely free from www.avast.com, Avast Home Edition antivirus software also provides anti-spyware and anti-rootkit protection, as well as P2P (peer to peer) and IM (instant messenger) shields for protection against malicious code sent via file sharing and messenger apps. Use of Avast Home Edition, while free, requires registration.
Microsoft Security Essentials – this free tool from the makers of Windows is available free to anyone using a legal and licensed version of Windows XP, Vista and 7. Designed to visually integrate with the operating system, Microsoft Security Essentials (available from www.microsoft.com) operates unobtrusively, providing a good level of protection from viruses.
ESET NOD32 - available for around $40 (or £29.95), NOD32 is marketed as the ideal antivirus for old operating systems such as Windows XP and the designers pride themselves on its fast scanning speed, remarkable considering it requires less memory and CPU resources than other premium products. Visit eset.co.uk for more information and to purchase NOD32.