Same with mine, an Acer 2.8GHz Celeron (Northwood) made in 2005, never bothered changing the CPU for a proper P4. However, this one has had a few minor upgrades since stock. Notably 256MB DDR1 became 1GB, 40GB IDE HDD gained a much larger friend (it's still the C: drive and has clocked up over 66000 hours), onboard SIS graphics was made redundant by a Radeon 9550 256MB. A few years ago, the power supply died so I had to swap that out as well (Teapo caps tend to do that, although somehow this one lasted 7 years). Not worth recapping a 200W unit though so it was scrapped for parts (a bridge rectifier and a few MOSFETs is worth the effort).
As for shutting down every night, this PC is still as fast as day one as well, yet it is only ever "shut down" when the power goes out! Myth debunked.
The best way to keep them fast is to not install junk like free screensavers and crippleware apps (all of which tend to be distributed in seedy, proprietary, EXE-based installers which may or may not install even more junk depending on whether it lets you opt-out or not, or they simply install them anyway). If you're a point-and-click casual user, buy a Mac or an iPad instead, no Win32 malware will dare invade your machine!
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Re: Keep XP but install Win 8.1 onto another hard drive.
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