what do i need to upgrade to make my comp faster?

i have vista home basic..compaq presario...system typex86..amd athlon processor 3800+,2400 mhz, 1 core, 1 logical processor....installed physical memory 2.50 GB..total physical memory 2.37 GB.. available physical memory 1.11 GB....i know its not that slow of a computer for me because i dont use it for gaming i jus want to at least be able to DL windows 7 and i cant when i try it says i dont have enough memory and i dont have that many things installed..do i need more memory???...also sometimes my comuter will act starnge like the windows toolbar quicklaunch will open all the icons even though i have it set for it wouldnt be that way and aslo i have to refresh icons sometimes to make them reappear where there supposed to be..firefox doesnt open all the time the way it should and a window pops up telling me that it cant run and shows me a C++ which i dont think is to good ...do i have a virus ?? i checked it with various programs and it says no but i have a feeling i do ..