I can't speak for anybody else,

What's your desktop model?  I bought two Acer desktops and they work perfectly and setup 2 more for other people and they work perfectly with both Windows 8.1 and Windows 10.  If you post your computer model, I might be able to give you some tips.

Can you post your complete model # ?  Here's a couple examples of xc-603.  One is xc-603-ur10 with a pentium and the other is a xc-603-ub17 with a celeron.

http://www.amazon.com/dp/B00M12YUJC/ref=pd_lpo_sbs_dp_ss_1?pf_rd_p=1944687602&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=B00OGOMOE6&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=1QY7JPMWDEK4TYAY4FS1

I will tell you though that you have to get in to the core cpu's such as i3 or i5 or i7 before you get increased processing power. (And your CPU is not upgradeable on your computer model).

A simple thing you can do is swap your HDD for a SSD (solid state drive) to really speed your computer up.  It doesn't matter if you buy an Acer, Dell, or HP with a celeron or pentium, it's not going to be super fast.  But again, a SSD can really speed up most any computer.

The Crucial bx200 is about the cheapest 500 GB SSD (480 GB) that you can get.  I'm assuming that your computer came with a 500 GB hard drive.

Make sure you create your factory default recovery media before you upgrade to a SSD (if you upgrade).

http://acer.custhelp.com/app/answers/detail/a_id/29934/related/1/session/L2F2LzEvdGltZS8xNDY1MzU3NzkwL3NpZC9Ea0RVbXZTbQ==

The xc-603 does only have a SATA2 port, but swapping your HDD for a SSD, even with its SATA2 port is going to make it a lot faster.

Sometimes you have to get a bracket so the SSD fits properly in the place where the HDD was mounted.

http://www.amazon.com/Notebook-Drive-Mounting-Bracket-Adapter/dp/B00AYJFXIQ

Ok but make sure your internet is ok.

http://www.speedtest.net/

And make sure you don't have a virus.

http://windows.microsoft.com/en-us/windows/using-defender#1TC=windows-10