I often loose tracking of the guarantee of my stuff like TV, notebook, ipod, etc.

I solved this problem using RTM by creating a task list called "guarantee" and I create a new task for each product. The task due date is the day which the guarantee ends. I also put important information about the product like support number or vendor name as notes for the task.

When something needs repair I look at the "guarantee" task list if the product is still there.