It's neither. The computer is clean. For whatever reason CERTAIN task on my CERTAIN machine are SOMEWHAT faster on a g4 but day-to-day both "feel" similar performance wise.The 1Ghz eMac was made in 2003/2004, the MDD G4 2002/2003. Google reveals the Optiplex GX2740 is from around 2003/2004. Back then a P4 2.6GHz would be found a low to mid-range computer.
It's not surprising that a Mac from the same time period is faster than the PC. You're comparing PPC and Pentium 4 clock speeds, which is comparing apples and oranges (no pun intended). One of the reasons Apple (I believe) switched to Intel processors was because people would question buying a 1.5GHz Mac when for the half the amount of money you could buy a 3Ghz PC.
The Asus T100 is at the most a couple years old, so it should be a faster than a 10 year old G4. Usually what ruins a PC is malware and/or bloatware. I suspect that would be the case.