The tiny setup program give you the option of configuring your PC in host or client mode, or skipping installation altogether (in which case TeamViewer runs without installing anything). I chose the latter option on two PCs: one running XP, the other, Vista. From there I simply entered a numeric ID and password generated by the software, and presto: Instant remote access. In addition to controlling the other PC, I could transfer files and chat with the other user (myself, in this case). By enabling server mode, I was able to switch directions and share my desktop with the other PC--neat for showing presentations, demonstrations, etc.
TeamViewer works around firewalls and promises 1024-bit RSA private/public key exchange and 128-bit RC4 session encoding. In other words, it's pretty secure. It also has at least one advantage over each of the three aforementioned remote-access programs, so if you're interested in connecting to another computer from afar, give TeamViewer a try. The program is free for personal, non-commercial use, and it requires Windows 98 or later. Thanks, Karri!