I restored the retro PC I had in childhood.

It’s equipped with a Pentium 4 Northwood on an IBM 49P4384 (NetVista) motherboard, 768MB of SDRAM, an ATI HD3650 512MB AGP card, an Audigy 2 PCI sound card, a basic Adaptec SCSI controller, and a TV tuner.

For system there is Windows XP SP3 with all updates from Legacy Update; I previously had One Core API installed but uninstalled it.

Performance in CPU-bound games is just terrible. Warcraft 3 v1.00 runs on lowest settings with 80x600 at around 20-something FPS while maxing out CPU usage (100%), while all other patches (1.07, 1.12, 1.26, 1.27) run at an around 15 FPS. Half-Life 2 also causes 100% CPU utilization and is unplayable even in "potato mode" (using a custom config file).

I initially thought the system was just weak on the CPU side, but Warcraft 3 suggests that this behavior likely isn't normal. What could be the issue, and what are the chances of fixing it without reinstalling the OS?