AVG is a good free anti-virus program but can tax your system resources. Most computers running Windows XP currently have a fraction of the memory that newer computers do. Two other alternatives are Avast! Anti-Virus and Microsoft Security Essentials.
Avast! Anti-Virus does not tax system resources as much as AVG Anti-Virus. Microsoft Security Essentials comes from Microsoft foray into the free anti-virus/spyware market. Like Avast! Anti-Virus it is lightweight but Microsoft Security Essentials does leave some questions open with regards to its effectiveness.
I would also recommend installing Malwarebytes Anti-Malware as a general virus/spyware/adware scanner in addition to a real time anti-virus program.