Newly Reformatted Computer / Painfully Slow

I recently reformatted my computer, after getting a virus and deciding that the effort it would take to salvage my computer wasn't worth it.Prior to getting a virus on my computer, it was very fast and didn't have problems with RAM or memory. After reformatting, however, my computer is very slow despite not having installed even 10 programs.It's not slow in that the computer is laggy (though it is sometimes, but that's not really something that concerns me), but that programs take very long to load. For instance, I'll open up Firefox, and Windows Task Manager shows the memory usage of firefox.exe increasing by increments of 50. So it'll take 2/3 minutes to open.In addition, installing new programs takes much much much longer than they should. For instance, it took me nearly an hour and a half to install a trial version of Photoshop CS3, only for it in the end to have a problem telling me to reinstall it. Windows Update took very long as well (nearly half a day!)

I haven't done anything different to this newly reformatted computer except for one thing the first thing that I did when I reformatted my computer was to install Trend Micro Anti-Virus, from a trial that I found online.I do know that prior to installing that program, my computer wasn't too slow at all... But sometime afterwards, it became very sluggish. I don't know if the issue relates at all to the antivirus program itself, but I thought you should know that my computer didn't start out slow.