I am unable to open RAW digital photos on my PC. The PC is fitted with Pentium i3 processsor with 4 GB of RAM. Please adivse me.
It depends what your goal is for the image. If you are planning to edit them, then the answer will be likely be different than if you are simply trying to view them.
That said, RAW actually refers to a group of different proprietary formats, and each manufacturer has a different format, with the details being specific to the camera model. More technical details about RAW formats have also been discussed in another answer.
There are many different conversion tools, some from the manufacturers, and some from third parties.
Some other questions have addressed some specific scenarios:
RAW is not a standard image format, it is the data that is captured by the sensor of your camera. It is not an image, but rather data points. Specialized software that understands how your camera's sensor captures data must be used to interpret that data and produce a meaningful image. The disadvantage to the format is that if you do not have such software installed, you will not be able to open the file. The advantage is that if you do, you can take much better control of how the image is produced.
Software such as Lightroom, Aperture, Darktable or the RAW processing software from your camera manufacturer should allow you to open the file and choose how to convert it to a normal image. Photoshop with Adobe Camera Raw is also able to do this. There are also a variety of other viewers (which don't provide manipulation or possibly only access a preview image that is embedded in the RAW) and editors (which allow you to develop it to a normal image) available.
A number of options are available to view raw format images. Picasa is free software solution. A low cost option is ACDSEE which works well for a quick review of the image. If you want to batch process many raw images then DxO Optics Pro is top end software that will give a high quality rendering of the raw image. Here is a link for a comparison of three current raw converters.