Thanks a lot. I've made a simple check with Mage::registry('current_category'); and then I've just edited the getSetIds variable. But how could I automate the process and apply the same method for all categories and search page also? I can't loop through all getProductCollection() because there are lots of products, simple and grouped, from different categories. Any advices?

Thanks a lot. I wasn't getting the trick with the attributes and I was wondering why my subproducts are not appearing when i search them in the Associated products page. Thanks again.

Thanks for answering my question. I will try your workaround in a couple of days and I'll get back to you with some news.

I'm not sure. I've installed many extensions, but smaller ones that don't have anything to do with categories.

Well, I'm not using flat catalogs. They're set to 'no'. And I'm not sure if the dissapearance coincides with reindexing, I would say not necessarily. I have only 4-5 categories, but I use only 2.