Recently, whenever I open a new program (be it Windows Explorer, a web browser, or whatever) with iTunes playing in the background, the music goes all funny. It slows down and the sound messes up. I can't explain it. The problem isn't experienced with any other music software though...

iTunes is one of the more resource heavy music playing programs.

What music are you listening to..

I've tried it with multiple tracks. When I open a program such as Skype when iTunes is playing music, it slows down, judders and skips.

Most music players don't cache the entire song in system memory. They use a buffer of so many milliseconds, load that amount of song at a time, and load the next so many milliseconds while what's currently in the buffer plays back. This uses some amount of your CPU, hard drive, and memory depending on the player, the song file, and the buffer size.

When you're loading a program, that program uses all the same bits of hardware to launch. Depending on your system, the hardware might not be able to flawlessly keep up with the task of loading and unloading the buffer while also loading a program, especially if the buffer is very small. Then even a slight hiccup will cause a delay between loading and unloading it.

The best solution would be to increase the buffer length. That means the player loads more of the song at a time, so if there's a hiccup while loading the next buffer there's still plenty of time for it to catch up to the playback. I don't know how to do that in iTunes because I will never touch iTunes.

Note that with a long buffer, there may be a delay between tracks if you're using some sort of random playback order, or when you manually select a track. That's because it has to fill the whole buffer before it plays. This should only matter with very high bitrate files.

I know it is, but I never used to have this problem. It "lags" even when just opening a web-browser...

Does anyone know? I am also currently playing using Direct Sound. Will switching to Windows Audio Session or whichever perhaps improve this?

I just checked and my CPU and RAM is fine? Not maxing out when opening new stuff.

It doesn't have to max out, it just has to slip up. Just like how if you launch two programs at once, they aren't likely to load as quickly as they would individually, even if the CPU is up to the task. Only this time your music is trying to play back almost in real time, so the margin of error is much more noticeable.

It's an issue of managing many instructions at once. Certain instructions take priority.

The buffer may not even be your issue, but it sounds like it from my experience.

Did you really have to triple post, though? Maybe you should try something yourself instead, like a new program to see if it works without issue. Give us more information to go on.