Especially with a low-end CPU and limited memory you are doing a lot of swapping to a slow disc. That will play havoc with any timings as soon as more than one app is running....

To run realtime apps successfully in Windows you need a fast CPU and lots of memory to minimise swapping. You need to harden Windows by removing all drivers you don't actually need - audio, graphics, modems, networks, CDs. You can create different hardware profiles that enable or disable specific hardware and load of apps. So you can have an 'on network' profile with network, cd, AVG and a 'CNC profle' with all these turned off. You disable all power management, although sometimes older machines don't let you.

Better still use the embedded version of Windows which doesnt swap and runs in memory - Most apps will run OK in it as long as they have been written well.

Then again, why mess around, you can pick up a good Pentium4/Windows XP desktop on ebay for £50 or so...