...especially at startup. The more items you have running, the more memory/resources you have used.. Uninstall those that you don't need. Try shutting down unnecessary startup programs in "Services" and possibly "msconfig". Unfortunately, there are lots of things that could be causing the issue..
Although AVG isn't a bad antivirus program, it won't find everything.. Try downloading, installing, updating, and running full scans using both of the free malware scanning tools below. Delete anything they find.:
Hope this helps.
I have a HP with Windows 7 on it that I got about 4 years ago. It has 4gb of ram in it and a decent video card that I added to it. While I have AVG up to date and it detects no viruses, my computer runs excessively slowly. It can't even handle youtube videos. I do disk defragmenter and cleanup regularly and have a lot of free space on my hard drive. Do you have any ideas what it could be?