Clean install Windows XP

My wife had bought a used computer Dell Optiplex 170 L, with Pentium 4, 2.8 Ghz, 512 RAM. It was one of many computers from an office and all the hd's were wiped clean, so no OS on any of them. However, each of them came with the OS in plastic to install ourselves. The lady who was selling it said that everything was in working order before removing the OS. Anyway, after reading up a little on how to do a clean install of windows XP on this Dell I started the process and about 12 or more hours later everything was initialized and it reboot. BTW, I did a regular full format of the HD. So I tried to connect to the internet and got nothing. Went to Help menu and only part of the help window was showing, not to mention that with every mouse click it was sooooooooo agonizingly slow to respond. I restarted and it still did the same sluggish performance. I thought for sure that by doing a clean install that it would be like having a new system and would be very fast. I went to Dell's website to support and downloads and dl'd the network driver since I noticed a yellow question mark in the network devices portion of my computer. I finally was able to get online albeit very slow still. Then I attempted to download the rest of the drivers and updated software and hardware info from Dell's site though it's Dell PC Tuneup program. Then I also went though a Windows update session thinking that it would only help matters. Anyway, here I am 2 weeks later and have the same slow performance on the computer. It is sooo dang slow. I can't stand it. Any suggestions? I even opened up the unit to make sure everything was secured properly and the way it should be. I'm tech savvy enough, just not good enough to figure this one out. Thanks.