Connectivity issues......

I apologize in advance if my post gets long winded....
I have a dell dimensions 2400 computer,Pentium 4 processor,running xp pro,2.66ghz with 640mb of Ram....I am using a cable modem via USB cable to access the internet....
I leave my computer on at all times,(I was told its better to leave it on "sleep" mode then continue shutting off and restarting) Anyways, In the morning the computer seems slow loading when I log into XP,if that loads slow I know I will have problems getting online, after I log into XP my desktop will load,but when I click on my internet explorer button or even my outlook express nothing will happen.Ive called the cable company and they run test on their end and they say my modem is responding to them,and the problem is on my end.Mind you this problem is intermittent doesnt happen all the time....Since this has started happening I have notice more "page wont load" or websites that I frequent several times a day not load for me. Also its gotten to the point on sometimes the computer just freezes...
I run AVG antivirus,CWShredder,Ad-Aware,Spybot....Plus I have a firewall up...
When this problem happens(from my desktop not opening any websites or email)If I completely shut down the computer wait a few minutes and restart there is a 50/50 chance that when I log back on XP I wont have the same problems....

My cable provider has basically said they have done everything they can on their end(including a new modem) and the problem still occurs.

I think my AVG is set to test everyday, and I run my AdAware and spybot at least once a week...I do "disc cleanup" and defrag at least once a month....


Any Ideas on what I can do,Ive had the dell for about 1 yr and this is my first problem I have had...
I appreciate any thoughts or suggestions on the matter....

For what its worth,I had the same problems last night(restarted twice) and havent had to much trouble today,however I went through and deleted temp internet files,cookies, disc cleanup, defrag, AVG test, AdAware,Spybot, and I downloaded the hijack this file....But I need some help here before I use the HJT file.

Thanks in advance.