Personally, I would recommend using either Avira AntiVir Free/Personal edition
for an antivirus program, just be sure that you only have 1 anti-virus program installed on your machine
For a firewall I would recommend Comodo
, please note that the Comodo firewall installation also includes an Anti-virus program, if you decide to choose another anti-virus (not Comodo), then please be sure to only install the firewall, you will be prompted during installation if you want to install just the firewall, or the firewall and the anti-virus.
The on-demand anti-malware scanners I recommend are Malwarebytes Anti-Malware
For additional protection, SpywareBlaster
is a great program to protect you from visiting malicious sites, and blocking tracking cookies. Note that SpywareBlaster isn't an actively running program. You download it, run it once, and then make sure you update it every week or 2 and you will always be protected. It works by tweaking some IE and Firefox settings to block tracking cookies and provide additional protection.
Even though you are running XP on a Mac, these programs should work, I have no experience with Parallel machine but am only making an educated guess.