I am running Windows XP in a VM. I want to download a few applications and install the one by one and check if they potentially can be a virus. I assume virus would need to add something to the startup folder, or the application in the startup section in the registry or add a service. What else might it do to become active?

Anyway, how can I check to see if a program may be a virus? I use hijack this to get a list of processes and I simply compare it from before I installed to after and see if there's anything different. Is this good enough? My main OS is Windows 7 but I do not have that in a VM and don't see a reason to test with that.