For Vista: Administrative Tools -> Computer Management -> Disk Management: If there is a hidden partition, you should be able to see it there. Those "64GB" SSDs are actually ~60 GB, hence I´m really wondering if your X300 has a hidden partition. But you should be able to verify this with the method above. Alternatively, you can press the ThinkVantage button in the very beginning of the boot process, this should bring you to the Rescue&Recovery predesktop area, then choose some of the recovery options there and it should respond (you don´t need to start the actual recovery process). If some of those steps fail, then either the predesktop area is broken or you don´t have the hidden partition.
To your question: AFAIK, yes, it´s possible to install XP and Linux in dual-boot configuration and in the same time keep the hidden partition. You simply need to be careful with your preferred partitioning utility. My experience is that the XP installation program doesn´t like multiple partitions (I don´t count the hidden one here), so here is how I see you should proceed:
1. Verify the existence of the hidden partition
2. Install XP on the Vista partition -> if needed, format only the Vista partition
3. After having installed XP, install Rescue&Recovery to restore the predesktop area (AFAIK, that´s the only way you can use the hidden partition for recovery purposes), in my experience if R&R disappears, then so does the predesktop area (or becomes unusable). However, I can´t tell for sure if that will work. But even if you manage to restore it (provided it´s gone after installing XP), it might not remain bootable after the following steps.
5. Use some universal partitioning tool to shrink the windows partition and create the linux partitions on the remaining
free space.
6. Install linux on the already created partitions.
The tricky part of all is actually not to keep the hidden partition, but to keep/restore Rescue&Recovery predekstop area. Because otherwise the hidden partition becomes simply unusable (to my best knowledge).
Apart from that, IMHO the best thing you can do is to
create recovery CDs after the first boot into Vista Rescue&Recovery. The you could wipe out the hidden partition, thus freeing up some space, and you will be able to restore the ThinkPad to its factory contents (Lenovo preloaded image of Vista) at any time you want.
Hope this helps, and btw welcome to our forum!
Marin