It seems likely your Windows XP is less secure than it was a few days ago, doesn't it? Given that the source code has been leaked and no-one is working on actively maintaining the code and patching security holes?
Not really. The biggest malware makers (governments, TLA agencies, major criminal organisations) will have had access to the source for more than a decade (with governments, since its release). If anything, it might improve security if it makes it easier for third parties to produce patches and enhancements.
Default XP installs are going to be vulnerable no matter what, but locked down setups which disable the most exposed elements (SMB/CIFS, IE/OE, etc) and use third party software to restrict access aren't likely to see any change. Plus having the source available for GNU/Linux or xBSD doesn't seem to have impacted their security in most users' eyes.
If anyone starts distributing unofficial patches to XP based on leaked source code, I would expect the MS lawyers to come down on that pretty hard ...
If anyone includes
MS code outside of its licensing (bear in mind that plenty of Windows code can be bundled with an application, like C++/DirectX redistributables or necessary Windows updates) then yes, Cease and Desists will start flying. However there are many ways to implement changes (patch files, diff files, DLL injection, drivers) without including a single byte of Windows code.
And if original code is being distributed, "proving" that it relied upon knowledge of leaked source code would require lawyers (and court judges) capable of mind-reading.