You say:
40GB hard disk.
That's where you start then.
How is the drive partitioned?
Is windoze on c:/ and you have a free drive d:/ and e:/ ?
This would be this simplest scenario, but not the most likely.
You have to partition your existing drive (or add a new one)
Adding a new drive is the easieast solution.
If windows is formatted to take it all, and you don't have the new drive option then you would be best to re-install windows with partitions.
If you do have a free partition on say the e:/ drive, that you feel safe to blow away; this is great! Start the linux install to reformat that partition. Hope you can recognise it by size or format first.
It wouldn't hurt to think about what you want Linux to do- not critical on first install- but you might want to start thinking about partitions. (if you end up loving it, it may be to your benefit
Bottom line: If you can't designate a part of your hard-drive for Linux you will have to add a new one.
Once added, Linux will be very friendly and recognise the other OS.
When asked on the Linux install, say yes install boot to MBR. (master boot record)
Have fun
linux and windows xp in a 
