|Some programs do things such as download files or update themselves when you are not actively using the computer, but it is rare for them to use that much of the cpu time (unless your Windows partition is too full - see below). |
(e.g. anti-malware programs, Window's own Automatic Updates, other programs that update themselves in the background).
You may have malware (a virus or a trojan or a worm, etc.), or, rarely, some bad guy is accessing your computer.
Is there a wireless connection between your computer and another networking device? If so, is the connection between them protected with security settings - encryption or a filter in their configuration - if NO, anyone within range can access your computer.
Do you have other symptoms? - if you do, we need all the details you can supply.
Your problem is probably caused by something that is using rundll32, not by rundll32 itself, but which program that is may not be shown in Task Manager - that's a common situation with malware, but it often isn't malware that is using it.
Trying to end the rundll32 process doesn't work more than temporarily because it's something else that is using it that you probably can't end the process of in Task Manager because it doesn't appear there.
If the problem is caused by something that is accessing the computer from the internet, or by something on your computer that needs to use the internet connection, if you unplug whatever cable that connects you to the internet, the program that is using rundll32 will immediately or eventually stop using the cpu time for rundll32 as much.
Try booting into Safe mode -
- remove any bootable CDs or DVDs you have in drives
- repeatedly press F8 while booting, starting early in the boot, do not hold down the key
- select Safe mode, or Safe mode with networking (if you need to update your anti-malware software), from the boot choices menu that pops up.
- try running a FULL scan with your anti-malware software.
- you MAY need to unplug whatever cable that connects you to the internet while doing that.
If your Windows partition, as in, the partition Windows itself was installed on, which is usually C, is too full, everything in Windows will slow down considerably, and processes that normally don't use an excessive amount of cpu time will likely use a lot more. There needs to be more than enough freee space on the partition for both the program's use and Window's use - some programs require a lot more free space than others do. E.g. People that have brand name system computers that still have their orginal software installation on them and have just the typical two partitions on the one hard drive that came with it, C and the Recovery partition whether it has a drive letter or not, who post here sometimes have that situation.
If you have this problem you will have noticed Windows runs considerably slower than it once did for the same things, all the time.
What's the size of the partition Windows is on, and how much free space is left on it?.