Well, you're doing what you can s/w wise. Though, I would recommend you keep the .dot framework s/w, that's required for better MS connections, etc.. Since, its present it will probably request itself again to reload via any MS updates and if running XP(updates) will no longer be supported.
As for the Dell 4550, you need to recheck the specs and what you have now. If this is socket 775 based, then you maybe able to replace the cpu, itself. Compared to what you now have, buy similar yet better cpu. Though, I strongly suggest you do a bios update to allow the max. latitude in accepting any newer cpu and/or range of cpus available. The max. cpu it states it can support is 3.06Ghz. Notice, that its a Intel 845PE chipset which via Intel can be updated as well or as another help source, plus the cpu comparison. You can't do better than what it states in normal setting. Anything is up to you and any daring setup. So, if you have a 2.2ghz cpu now, then a 2.4Mhz won't be a big improvement, but the 2.8Ghz or better should see something better than what you had. If you have any Celeron, a full P4, is bound to be better. Of course, having the max. ram should help as well. If all this is cost effective(used items) go for it. Otherwise, spend you $ looking at newer PCs. Used dual-core should be reasonable, check eBay or Craigslist for pricing.
My computer runs very slow. My AV program"s quick scan takes a half hour, minimum. I want to get anything off that I don't need and might be slowing it down. I'm not a gamer, I don't run a lot of software, Right now I'm trying to determine if I need the dot net Framework software (I'm leaning heavily towards NO),, just thinking that whatever I can get off this dinosaur will be an improvement. I've changed my AV (dumped McAfee) and shut HP down in task manager. I suspect Itunes is killing me now, but I don't know. I'm afraid my old Dell Dimension 4550 (pentium4) is the real problem ....am I right ?