First of all, welcome to the forum.
I did a Google search, and found this hardware:
Intel Pentium M 735 (1.7GHz) Processor
512MB DDR2 Memory
60GB 5400rpm Hard Drive
24x CD-RW/DVD Combo Drive
15.4" WXGA (1280x800) Wide Screen LCD
Intel GMA 950 Integrated Graphics with 128MB Shared Memory
v.90 56Kbps Modem, 10/100 Ethernet and 802.11b/g Wireless
Three USB 2.0 Ports and Express Card/54 Slot
This type of system will run most linux distros. I have a similar desktop that came with XP home. I have Slackware 13.1 running on it. The things I can not tell you about are the wireless adapter, or the modem. If your use is 'at home' the ethernet connection will work out of the box 99% of the time. To help with those, we need to know the 'chip set' of each device. Most wireless can be made to work, with some effort.
There are other distros I would suggest you try, since you have little experience. Slackware is not the easiest distro to start with. A short list : Magia 1, Ubuntu, or Mint linux.
The HD is on the small side. A lot depends on how much space you want/need to keep for XP, and how much you can make available to a linux system.
If you decide to repartition, you would need about 8 gig for the root file system, a 1 gig swap, and what ever you could afford for /home.
If you do not want to re-partition the hd, you could look into buying a new, larger HD. they are not that difficult to change. You could then start with a new partitioning scheme.
Last option would be to install Ubuntu, with Wubi. This installs Ubuntu on your existing HD using the existing file system. I have never used it, since I would go get a new HD with that hardware. My older system has a 120 gig disk. I re-partitioned, and installed.
External disk drives. I'm not familiar with the specific one you have. That said, most of them come formatted with a windows file system, either Fat32 or NTFS.
Linux uses different file systems, the list is very long. Linux, today can read and write Fat32 and NTFS. However, those systems do not support linux file permissions, and I would not recommending installing to either file system. Look into the WD and see what file system it has. You should be able to tell from XP.
Hope this helps.