Abbey Road's drum samples are quite poorly set up in Kontakt compared to other libraries.
First of all though, if you are using Kontakt within a DAW as you are (as opposed to outside a DAW and wiring it in through Plogue Bidule or another Virtual Instrument Host) - DO NOT enable the multicore processing.
You want your DAW to have multicore processing enabled and NOT Kontakt. This is something that multiple composers I know have done for a while on both Mac and PC platforms and it works.
(You can do it the other way round, have the DAW do single processing and Kontakt multicore, but I wouldn't recommend it. The point is, don't do both. You will find hundreds of forum posts about this online).
Another trick to get better performance from Kontakt inside a DAW is to not load tons of instruments in one instance of Kontakt. It's better to have multiple instances of Kontakt, then 1 big instance having 16 (or more) instruments in it. Don't ask my why, but most DAWs seem to handle Kontakt better this way.
However, the biggest problem in your specific case is not actually Kontakt, it's Modern Drummer itself (and the other NI Drummer programs). I'm running a Windows 7 64-bit computer with a top Intel processor, 32 GB of RAM and all samples streaming off SSD's....and Modern Drummer still eats (not kills, but eats) CPU. The CPU usage is way too high for just a drumset, especially compared to Superior Drummer / Addictive Drums or EZDrummer. So why is this?
The reason for this, is because there are so many ridiculous effects and EQs (which NI have put quite a lot of time and effort in to) on the drums. These can really make the drums sound great, but the effects - especially those inside Kontakt - have always been horrible CPU hogs. I don't touch them.
It is the effects that are giving you CPU spikes, not RAM or hard disk problems. Low RAM may be a factor, but it's not the main one. The effects and your multicore CPU usage as mentioned earlier is what's giving you the problems. I'm 99% sure of that.
I'm going to illustrate this effects thing right now for you. Here is the kick drum effects channel in Modern Drummer on one of the 'mixer' presets - Soul - RnB.
Look at that! You have at the bottom lit up: a compressor, a transient master, and an EQ. And these effects are all pretty high grade effects (by NI standards). And that's just the Kick Drum!!! These are all killing your CPU
Let's continue with the snare drum!!
We also have the transient master, compressor and EQ on this as well. Woohoo, More CPU killing going on.
And it continues like this with all the drums.
But, I'm not finished. There is almost certainly going to be some reverb on some of your presets too...
Mmm...yes.... a bunch of pretty pictures showing off NI's mediocre convolution reverbs, probably all preloaded into the patches (this is the case in the Kontakt factory library too. Watch your CPU die a quick death once loading a few Kontakt orchestral library patches. It loads a new reverb EVERY TIME you load a new patch. Reverb = kill CPU).
You can probably see my point right now. I've given enough examples. However....
....Just to prove my theory that it's the inbuilt effects that are killing your CPU, load up the 'INIT' preset in the Mixer presets window. Here is a picture of where to get that.
Once you've selected that, try playing a few notes. I bet your CPU usage disappears .... but!! ... your drums are gonna sound a bit crappy.
Native Instruments Modern Drummer is a really awesome piece of kit, but you're going to need a decent CPU to have it running with all the bells and whistles. A solid state drive will certainly help, but it's the EFFECTS that are killing people on this. The sample pool is not that large and so more RAM or a better disk isn't going to help things the most. (But it will help).
I would implore you to switch to an Intel CPU as soon as you can. I know it means getting a new motherboard as well, but Intel are so far ahead of the game I don't understand why people buy AMD chips anymore.
However, I'd also implore you if you don't want to change your CPU, to make sure not both Kontakt AND your DAW are doing multicore processing. I'm not guaranteeing that this will work, but it may help.
I'd recommend more RAM (you can never have enough), but in this specific case, that shouldn't be what's holding you back. Your DAW does not take up 4 GB of RAM when you first load it up (usually between 75MB - 250 MB depending on the DAW), so you should have more than enough RAM to deal with one instance of Modern Drummer / Kontakt 5 Player.
Finally, you're still using Windows XP? Time to get Windows 7. It might be a painful upgrade, but it's super stable and great for audio.It's not that Kontakt isn't compatible with XP, it's just that NI developed Kontakt 5 primarily for Mac OSX and Windows 7, so it's going to work best on those platforms.
I hope all of this helped.