Even with 512MB of RAM I would not have expected a memory issue pasting anything into Word.
I think you mean Paste don't you. You copied something from somewhere else, to 'paste' into word. What was it you copied into Word? I don't mean in detail, I mean was it an image or text? How large was it? I know you can't tell exactly, but was it a large image, or was it pages and pages of text?
Memory, (RAM), may have been affected if the data copied was very large, and you had other applications open at the same time.
The other possibility is disk space. That's the hard disk, which is very different from memory. If your hard disk is nearly full, then you could be having disk space problems and this could, strangely, be causing memory problems because a certain portion of the hard disk is allocated to 'Virtual Memory'; but if the disk is full, the virtual memory, (also called the Paging File), will disappear.
Please check your disk space. Open up your My Computer from the icon on the Desktop or in the Start Menu, then highlight and right click your hard disk, called the "Local Disk (C: )". select Properties. That will tell you the used space and free space. Tell us both of those please.
About your RAM. I also would advise upgrading to 2GB, (in total), if you can afford it. The details you gave means that you could only have 3GB in total, (3072MB). But don't go that far, anything above 2GB is not needed and not used.
You have 4 slots where the RAM can go, and in one of them you already have 512MB. So, I would advice you go for 3 more 512MB modules, making 4 512MB modules in total. 4 x 512 = 2048MB or 2GB
Let us know the details above, and good luck with the RAM. You will notice a vast improvement.