|"you are almost out of virtual RAM warnings after an hour of use"|
More than likely your hard drive is almost full.
"why do old PCs slow down anyway"
Mostly because people don't maintain them properly, both on the software side AND the hardware side of things. For instance, they let the hard drive get almost full, lol. Also, over these past 10 years, how many times have you opened the case & cleaned out all the dirt & dust? Dust that builds up on the heatsinks & fans negatively affect the cooling causing the CPU to run hot. Once the CPU reachs a certain "trigger temp", it attempts to lower the temp by slowing down. The hotter it gets, the slower it will run. At some point, it will get so hot that it will completely shutdown to prevent itself from burning out.
Here's your system specs:
If you want it to run decently again, take the case outside, open it up, & blast the heck out of it with compressed air (do NOT use a vacuum). Once that's done, hook everything back up again, completely wipe the hard drive, then reinstall Windows from scratch using a retail disc (that way you'll avoid installing the Dell crapware).