18g may or may not be enough based on the setting on how much windows is looking for. If it's looking for 50 and you only have 18 for example, it will constantly tell you that you are low on virtual memory. Though that is plenty, you may just need to tell windows to look for less. To change what Windows is looking for (it's also possible since you say you have 18 gb on C alone that your system may be looking to other drives if you have them, check that too):
How to manually change the size of the virtual memory paging file
You must be logged on as an administrator or as a member of the Administrators group to complete this procedure. If your computer is connected to a network, network policy settings may also prevent you from completing this procedure.
1. Click Start, click Run, and then type sysdm.cpl in the Open box.
2. Click the Advanced tab, and then click Settings under Performance.
3. Click the Advanced tab, and then click Change under Virtual memory.
4. Under Drive [Volume Label], click the drive that contains the paging file that you want to change.
5. Under Paging file size for selected drive, click Custom size, type a new paging file size in megabytes (MB) in the Initial size (MB) or Maximum size (MB) box, and then click Set.
If you decrease the size of either the initial or maximum paging file settings, you must restart your computer to see the effects of those changes. When you increase the paging file size, you typically do not have to restart your computer.
Notes
• To have Windows select the best paging file size, click System managed size. The recommended minimum size is equivalent to 1.5 times the RAM on your computer, and 3 times that figure for the maximum size. For example, if you have 256 MB of RAM, the minimum size is 384 MB, and the maximum size is 1152 MB.
• For best performance, do not set the initial size to less than the minimum recommended size under Total paging file size for all drives. The recommended size is equivalent to 1.5 times the RAM on your computer. It is good practice to leave the paging file at its recommended size. However, you may increase its size if you frequently use programs that use much memory.
• To delete a paging file, set both the initial size and the maximum size to zero, or click No paging file. We strongly recommend that you do not disable or delete the paging file.
It's possible that something you are working on creates large temp files (like photoshop for example) that takes up most, if not all of the free disk space, so that windows doesn't see enough space. Auto recover files in Office documents, etc all take up disk space and you may not know they are even there when the application is closed. If you see it usually with one program running (for me it's photoshop) you may be able to adjust the size or location of those temp files to help.