The bootload on the LPC2xxxx is built-in and will never get erased. You can query the bootloader using a terminal program (such as HyperTerminal in Windows) and ask information such as product ID.
After reset, pin P0.14 of the LPC2103 is tested. If it's at LOW level for a few milliseconds after the reset line goes from low to high, the bootloader will run. And the UART0 (RXD0/TXD0) will wait for code from you PC.
The general concept of bootloading:
Set BSL jumper
Press the reset button
Start the flash downloader
The transfer of program from PC to the target board should begin. To start the program, remove the BSL jumper and press Reset.
In Windows, the flash downloader is Flash Magic (http://www.flashmagictool.com) from Embedded Systems Academy. It has a graphical user interface (GUI).
In Linux, use lpc21isp (http://sourceforge.net/projects/lpc21isp/). The command is lpc21isp program.hex dev/ttyS0 115200 14746
Another program in Windows is LPCProg 0.3 (http://home.arcor.de/bernhard.michelis/LPC/index.html) but the user interface is in German and requires NET 2.0 Framework and Visual C++ SP1 Redistributable.
If the flash downloader can control the DTR/DTS signal of the on-board UART, you don't have to set/remove the BSL jumper.