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Virtual memory (also known as a page file) is essentially a block of space on your hard drive or SSD being allocated by the OS to pretend to be RAM when your physical RAM runs short for actively running programs. It supplies additional "fake" RAM to allow programs to continue functioning, but because hard drive and SSD access and performance is much slower than that of actual RAM, noticeable performance loss is usually observed when relying on virtual memory extensively.
Managing virtual memory in a Windows system is relatively easy, but largely unnecessary. Windows will, by default, adjust it on the fly based on your needs and your installed RAM. It tries to set itself to what it expects you to need based on your installed RAM and it will adjust itself if your usage spikes. This can lead to it self-adjusting to end up taking a large amount of space if you are currently running a large amount of RAM (for example a system with 8GB of RAM will often default to 8GB of virtual memory and can grow as large as 16GB). Some users will change it to reduce space being used by their OS on their hard drive or SSD because they feel they have sufficient RAM to never need to rely on the virtual memory feature of their OS, and (especially with higher cost storage on an SSD) to reclaim space that is simply being wasted if that virtual memory is not being used. However, reducing or eliminating your virtual memory to free up drive space carries some risk of causing errors or system instability in cases where you would need more but have capped what your system can use.
Increasing this memory setting is also generally not needed. Instances where more virtual memory makes the difference between a program working or not are best solved with a RAM upgrade, as RAM speeds greatly exceed the performance of virtual memory. Your system performance will be reduced compared to improving the dedicated hardware meant to provide this benefit.
Manual adjustments to this setting are done at your own risk, and this should not be attempted unless you have solid guidelines toward what you actually need. Under-allocating space for this can lead to programs or your system as a whole malfunctioning. Additionally, older programs are designed with the assumption there is a page file in place for them to use, and malfunctions of individual programs may occur if this is unavailable to them due to incorrect configuration.
Detailed instructions for these adjustments in older versions of Windows are featured at http://www.crucial.com/support/pc_virtual_memory.a
1. Enter System Properties by either the 'System' link in the 'System and Security' sub-menu of Control Panel or by right-clicking 'Computer' in your start menu and selecting 'Properties (Windows 7 and earlier only). This will open the Window pictured below.
2. Select 'Advanced System Settings' (link outlined in red numbered 1 in the above screen). This will open a window titled "System Properties".
3. Click the "Settings" button in the Performance category (outlined button labeled 2 in the screenshot above). This opens the Performance Options window.
4. Navigate to the Advanced tab (outlined and labeled 3) then click the Change button (outlined and labeled 4). This will open the Virtual Memory window (outlined and labeled 5), from which any adjustments can be made.
If you disable the 'Automatically manage paging file size for all drives', this window will allow you to manually set sizes or size ranges, specify which drive this space is placed on if you have more than one (to free up space on an SSD by moving the page file to a secondary platter hard drive), or disable the feature entirely.
While some users do shut this off, our official recommendation is to leave it in place. If you are comfortable reducing the size based on your usage that can be done safely, but disabling this entirely is not necessarily safe. Even if you feel you have sufficient RAM to go without this, that change does put you at risk of poorly optimized software, particularly something with a memory leak issue ballooning it's RAM footprint, causing malfunctions later.