Paint.net
edit: I didn't see the part about your PC being old.
these are the requirements anyways
* Windows XP (SP2 or later),
or Windows Vista,
or Windows Server (2003 SP1 or later)
* .NET Framework 2.0
* 500 MHz processor (Recommended: 800 MHz or faster)
* 256 MB of RAM (Recommended: 512 MB or more)
* 1024 x 768 screen resolution
* 200+ MB hard drive space
* 64-bit support requires a 64-bit CPU that is running a 64-bit version of Windows, and an additional 128 MB of RAM
edit: I didn't see the part about your PC being old.
these are the requirements anyways
* Windows XP (SP2 or later),
or Windows Vista,
or Windows Server (2003 SP1 or later)
* .NET Framework 2.0
* 500 MHz processor (Recommended: 800 MHz or faster)
* 256 MB of RAM (Recommended: 512 MB or more)
* 1024 x 768 screen resolution
* 200+ MB hard drive space
* 64-bit support requires a 64-bit CPU that is running a 64-bit version of Windows, and an additional 128 MB of RAM