My pc is so slow and moody- task bar disappearing,freezing,etc?

i asked this question the other day but the whole question did not appear as i never added details ( i was using this website for first time) i have windows xp home sp2 i run comodo as my only firewall and avira free version as my anti-virus i also scan with superantispyware and until recently malwarebytes i can't afford a new computer as i have a 1 year old baby . does anyone know how to find out if my system is infected as something must be slowing it down and i've tried defragmenting,i also use c-cleaner regularly i do have a lot of programmes and services but i do not know what ones i need so am weary of removing them without being sure for instance in my list of programmes it says i last used avira in dec 2009 this cannot be right as i update each day and scan weekly any advice on how to fix this would be appreciated and which programmes&services do i actually need? thank you in advance and thanks also for the people who answered on monday, as i said the full question was not on there!!!