Lots of weird problems happen if you’re low on disk space–programs randomly fail, the operating system can seem unreliable, and you might lose work because you can’t save it.
Just like Windows XP, Windows Vista includes the Disk Cleanup Wizard. To use the Disk Cleanup Wizard to free up space, follow these steps:
1. Click Start, and then click Computer.
2. Right-click the drive you need to improve free space on, and then click Properties.
3. On the General tab, click Disk Cleanup.
4. On the Disk Cleanup Options page, click either My files only or Files from all users on this computer. You’ll clean up more disk space if you work with files from all users, but you need administrative credentials. If prompted, provide administrative credentials.
5. After a few seconds, the Disk Cleanup page appears. Scroll through the available options, and select the check box for anything you’re ready to delete. If you’re not sure, leave it.
You’re pretty safe removing the following files:
* Downloaded Program Files
* Temporary Internet Files
* Microsoft Error Reporting Temporary Files
* System error memory dump files
* System error minidump files
* Temporary files (close your programs first!)
* Per user archived Windows Error Reporting files
* Per user queued Windows Error Reporting files (hey, it’ll probably crash again in the future anyway)
* System archived Windows Error Reporting files
* System queued Windows Error Reporting files
Well, it never hurts to run a malware scan just to be on the safe side.
If you're getting low disk space messages, then probably, there isn't enough space on the drive for the paging file/virtual memory which might lead to problems.
So, as suggested above, running disk cleanup first would be immensely helpful. Then defrag.
As for free defraggers, you can download the free trial versions of commercial defragmenters from the vendors' site. These work just like the full version for the demo period (usually about a month), and are much more powerful than the default defragger. They can defrag in low space (which may be important in your case), defrag system files (page file, MFT, boot files etc) and get the job done quickly. The automatic versions are the best since they can be set once and forgotten.
Free ones, auslogics is good. Hop onto download.com and search for smart defrag. You can set it to autostart and run in your tray, then when you are idle, it will defrag for you when you've been off the computer for 5 minutes or more. But when it sees like 20% cpu usage, it turns off. Might be a good solution.