If I install the latest update, will it have any effecr on the existing websites on my system?
as long as your work did not require 'adding to XAMPP' (in the form of libraries or extensions) - your work will probably be OK. You will want to backup any MySQL or SQLite databases you have created along the way. Needless to say - also copy/backup your DocumentRoot(s) and any modifications to Apache conf files and make a copy of your php.ini to work from (do not try to 'copy it' into your new install).
This needs the following support: the GD library
Although it is not listed on the XAMPP for Windows 1.77 DL page:http://www.apachefriends.org/en/xampp-windows.html
GD support is built-in - you can confirm by running Instant Art or phpinfo() on XAMPP 1.7.7 Welcome page...
phpinfo() for XAMPP 1.7.7
GD Support enabled
GD Version bundled (2.0.34 compatible)
FreeType Support enabled
FreeType Linkage with freetype
FreeType Version 2.4.3
GIF Read Support enabled
GIF Create Support enabled
JPEG Support enabled
libJPEG Version 6b
PNG Support enabled
libPNG Version 1.2.46
WBMP Support enabled
XBM Support enabled
Directive Local Value Master Value
gd.jpeg_ignore_warning 0 0