Very slow boot and operation

My daughters PC was infected by trogens, viruses and many spyware objects. This caused her PC to work extremely slow and my attempts at removing these viruses etc. failed because they kept re-appearing. Having spent some days trying to do this I gave up and decided a complete new installation of XP was needed. This new installation has not helped at all and the PC is as slow as ever and if I install ZoneAlarm and Grisofts AVG anti-virus software it makes things a lot worse, infact it takes 10 minutes to boot to the desktop as opposed to the 7 minutes without avg and zonealarm installed.

The PC is not great spec I know, being 450 mhz processor, with 256 meg of RAM and an 8 gig hard drive, but even adding an extra 256meg of RAM makes no difference to the speed. I'm wondering could some of the viruses still be on the hard drive even after formatting and re-parbreastioning the hard drive prior to the new XP installation. If no can anyone offer any further suggestions please, I've also install SP2 which took 5 hours to do. I'm lost and need som help. The PC is around 4 years old and my daughter only uses it for the web and a few letters. Can you help, please? AL